assistant: Handle back event in web container

This patch updates the back button visibility for Assistant web
container.

Bug: b/141268413
Test: manual
Change-Id: I2dfd3209e4954e04295340d48f05516bde37d09a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873147
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712852}

ash/BUILD.gn

@@ -210,6 +210,8 @@ component("ash") {
     "assistant/assistant_view_delegate_impl.h",
     "assistant/assistant_web_ui_controller.cc",
     "assistant/assistant_web_ui_controller.h",
+    "assistant/assistant_web_view_delegate_impl.cc",
+    "assistant/assistant_web_view_delegate_impl.h",
     "autoclick/autoclick_controller.cc",
     "autoclick/autoclick_controller.h",
     "autoclick/autoclick_drag_event_rewriter.cc",

ash/app_list/views/assistant/assistant_page_view.cc

@@ -85,8 +85,9 @@ void AssistantPageView::InitLayout() {
 
     // Do not add web view when Assistant web container is enabled.
     if (!chromeos::assistant::features::IsAssistantWebContainerEnabled()) {
-      assistant_web_view_ = AddChildView(
-          std::make_unique<AssistantWebView>(assistant_view_delegate_));
+      assistant_web_view_ = AddChildView(std::make_unique<AssistantWebView>(
+          assistant_view_delegate_,
+          /*web_container_view_delegate=*/nullptr));
     }
 
     // Update the view state based on the current UI mode.

ash/assistant/assistant_ui_controller.cc

@@ -399,7 +399,8 @@ void AssistantUiController::OnDeepLinkReceived(
     assistant::util::DeepLinkType type,
     const std::map<std::string, std::string>& params) {
   // This method only handles web deep links, which will be handled separately
-  // in |AssistantWebUiController| when Assistant web container is enabled.
+  // in |AssistantWebUiController| when Assistant web container is
+  // enabled.
   if (chromeos::assistant::features::IsAssistantWebContainerEnabled())
     return;
 

ash/assistant/assistant_web_ui_controller.cc

@@ -12,14 +12,65 @@
 #include "ash/shell.h"
 #include "chromeos/services/assistant/public/features.h"
 #include "ui/aura/client/aura_constants.h"
+#include "ui/events/event_observer.h"
+#include "ui/views/event_monitor.h"
 
 namespace ash {
 
+// -----------------------------------------------------------------------------
+// AssistantWebContainerEventObserver:
+
+class AssistantWebContainerEventObserver : public ui::EventObserver {
+ public:
+  AssistantWebContainerEventObserver(AssistantWebUiController* owner,
+                                     views::Widget* widget)
+      : owner_(owner),
+        widget_(widget),
+        event_monitor_(
+            views::EventMonitor::CreateWindowMonitor(this,
+                                                     widget->GetNativeWindow(),
+                                                     {ui::ET_KEY_PRESSED})) {}
+
+  ~AssistantWebContainerEventObserver() override = default;
+
+  // ui::EventObserver:
+  void OnEvent(const ui::Event& event) override {
+    DCHECK(event.type() == ui::ET_KEY_PRESSED);
+
+    const ui::KeyEvent& key_event = static_cast<const ui::KeyEvent&>(event);
+    switch (key_event.key_code()) {
+      case ui::VKEY_BROWSER_BACK:
+        owner_->OnBackButtonPressed();
+        break;
+      case ui::VKEY_W:
+        if (!key_event.IsControlDown())
+          break;
+
+        event_monitor_.reset();
+        widget_->Close();
+        break;
+      default:
+        // No action necessary.
+        break;
+    }
+  }
+
+ private:
+  AssistantWebUiController* owner_ = nullptr;
+  views::Widget* widget_ = nullptr;
+
+  std::unique_ptr<views::EventMonitor> event_monitor_;
+
+  DISALLOW_COPY_AND_ASSIGN(AssistantWebContainerEventObserver);
+};
+
+// -----------------------------------------------------------------------------
+// AssistantWebUiController:
+
 AssistantWebUiController::AssistantWebUiController(
     AssistantController* assistant_controller)
     : assistant_controller_(assistant_controller) {
   DCHECK(chromeos::assistant::features::IsAssistantWebContainerEnabled());
-
   assistant_controller_->AddObserver(this);
 }
 
@@ -56,6 +107,11 @@ void AssistantWebUiController::ShowUi() {
   web_container_view_->GetWidget()->Show();
 }
 
+void AssistantWebUiController::OnBackButtonPressed() {
+  DCHECK(web_container_view_);
+  web_container_view_->OnBackButtonPressed();
+}
+
 AssistantWebContainerView* AssistantWebUiController::GetViewForTest() {
   return web_container_view_;
 }
@@ -63,9 +119,12 @@ AssistantWebContainerView* AssistantWebUiController::GetViewForTest() {
 void AssistantWebUiController::CreateWebContainerView() {
   DCHECK(!web_container_view_);
 
-  web_container_view_ =
-      new AssistantWebContainerView(assistant_controller_->view_delegate());
-  web_container_view_->GetWidget()->AddObserver(this);
+  web_container_view_ = new AssistantWebContainerView(
+      assistant_controller_->view_delegate(), &view_delegate_);
+  auto* widget = web_container_view_->GetWidget();
+  widget->AddObserver(this);
+  event_observer_ =
+      std::make_unique<AssistantWebContainerEventObserver>(this, widget);
 
   // Associate the window for Assistant Web UI with the active user in order to
   // not leak across user sessions.
@@ -78,16 +137,16 @@ void AssistantWebUiController::CreateWebContainerView() {
   if (!active_user_session)
     return;
 
-  web_container_view_->GetWidget()->GetNativeWindow()->SetProperty(
-      aura::client::kCreatedByUserGesture, true);
-  window_manager->SetWindowOwner(
-      web_container_view_->GetWidget()->GetNativeWindow(),
-      active_user_session->user_info.account_id);
+  auto* native_window = widget->GetNativeWindow();
+  native_window->SetProperty(aura::client::kCreatedByUserGesture, true);
+  window_manager->SetWindowOwner(native_window,
+                                 active_user_session->user_info.account_id);
 }
 
 void AssistantWebUiController::ResetWebContainerView() {
   DCHECK(web_container_view_);
 
+  event_observer_.reset();
   web_container_view_->GetWidget()->RemoveObserver(this);
   web_container_view_ = nullptr;
 }

ash/assistant/assistant_web_ui_controller.h

@@ -5,8 +5,11 @@
 #ifndef ASH_ASSISTANT_ASSISTANT_WEB_UI_CONTROLLER_H_
 #define ASH_ASSISTANT_ASSISTANT_WEB_UI_CONTROLLER_H_
 
+#include <memory>
+
 #include "ash/ash_export.h"
 #include "ash/assistant/assistant_controller_observer.h"
+#include "ash/assistant/assistant_web_view_delegate_impl.h"
 #include "base/macros.h"
 #include "ui/views/widget/widget_observer.h"
 
@@ -14,6 +17,7 @@ namespace ash {
 
 class AssistantController;
 class AssistantWebContainerView;
+class AssistantWebContainerEventObserver;
 
 // The class to manage Assistant web container view.
 class ASH_EXPORT AssistantWebUiController : public views::WidgetObserver,
@@ -31,6 +35,8 @@ class ASH_EXPORT AssistantWebUiController : public views::WidgetObserver,
       assistant::util::DeepLinkType type,
       const std::map<std::string, std::string>& params) override;
 
+  void OnBackButtonPressed();
+
   AssistantWebContainerView* GetViewForTest();
 
  private:
@@ -42,9 +48,14 @@ class ASH_EXPORT AssistantWebUiController : public views::WidgetObserver,
 
   AssistantController* const assistant_controller_;  // Owned by Shell.
 
+  AssistantWebViewDelegateImpl view_delegate_;
+
   // Owned by view hierarchy.
   AssistantWebContainerView* web_container_view_ = nullptr;
 
+  // Observes key press events on the |web_container_view_|.
+  std::unique_ptr<AssistantWebContainerEventObserver> event_observer_;
+
   DISALLOW_COPY_AND_ASSIGN(AssistantWebUiController);
 };
 

ash/assistant/assistant_web_view_delegate_impl.cc

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ash/assistant/assistant_web_view_delegate_impl.h"
+
+#include "ash/frame/non_client_frame_view_ash.h"
+#include "ash/public/cpp/caption_buttons/caption_button_model.h"
+#include "ui/views/widget/widget.h"
+#include "ui/views/window/caption_button_types.h"
+#include "ui/views/window/non_client_view.h"
+
+namespace ash {
+
+namespace {
+
+class AssistantWebContainerCaptionButtonModel : public CaptionButtonModel {
+ public:
+  AssistantWebContainerCaptionButtonModel() = default;
+  ~AssistantWebContainerCaptionButtonModel() override = default;
+
+  // CaptionButtonModel:
+  bool IsVisible(views::CaptionButtonIcon type) const override {
+    switch (type) {
+      case views::CAPTION_BUTTON_ICON_CLOSE:
+        return true;
+
+      case views::CAPTION_BUTTON_ICON_BACK:
+        return back_button_visibility_;
+
+      case views::CAPTION_BUTTON_ICON_MINIMIZE:
+      case views::CAPTION_BUTTON_ICON_MAXIMIZE_RESTORE:
+      case views::CAPTION_BUTTON_ICON_LEFT_SNAPPED:
+      case views::CAPTION_BUTTON_ICON_RIGHT_SNAPPED:
+      case views::CAPTION_BUTTON_ICON_MENU:
+      case views::CAPTION_BUTTON_ICON_ZOOM:
+      case views::CAPTION_BUTTON_ICON_LOCATION:
+      case views::CAPTION_BUTTON_ICON_COUNT:
+        return false;
+    }
+  }
+
+  bool IsEnabled(views::CaptionButtonIcon type) const override { return true; }
+
+  bool InZoomMode() const override { return false; }
+
+  void set_back_button_visibility(bool visibility) {
+    back_button_visibility_ = visibility;
+  }
+
+ private:
+  bool back_button_visibility_ = false;
+
+  DISALLOW_COPY_AND_ASSIGN(AssistantWebContainerCaptionButtonModel);
+};
+
+}  // namespace
+
+AssistantWebViewDelegateImpl::AssistantWebViewDelegateImpl() = default;
+
+AssistantWebViewDelegateImpl::~AssistantWebViewDelegateImpl() = default;
+
+void AssistantWebViewDelegateImpl::UpdateBackButtonVisibility(
+    views::Widget* widget,
+    bool visibility) {
+  auto caption_button_model =
+      std::make_unique<AssistantWebContainerCaptionButtonModel>();
+  caption_button_model->set_back_button_visibility(visibility);
+
+  auto* non_client_view = widget->non_client_view();
+  DCHECK_EQ(NonClientFrameViewAsh::kViewClassName,
+            non_client_view->GetClassName());
+  auto* frame_view_ash =
+      static_cast<NonClientFrameViewAsh*>(non_client_view->frame_view());
+  frame_view_ash->SetCaptionButtonModel(std::move(caption_button_model));
+}
+
+}  // namespace ash

ash/assistant/assistant_web_view_delegate_impl.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_ASSISTANT_ASSISTANT_WEB_VIEW_DELEGATE_IMPL_H_
+#define ASH_ASSISTANT_ASSISTANT_WEB_VIEW_DELEGATE_IMPL_H_
+
+#include "ash/assistant/ui/assistant_web_view_delegate.h"
+#include "base/macros.h"
+
+namespace ash {
+
+class AssistantWebViewDelegateImpl : public AssistantWebViewDelegate {
+ public:
+  AssistantWebViewDelegateImpl();
+  ~AssistantWebViewDelegateImpl() override;
+
+  // AssistantWebViewDelegate:
+  void UpdateBackButtonVisibility(views::Widget* widget,
+                                  bool visibility) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(AssistantWebViewDelegateImpl);
+};
+
+}  // namespace ash
+
+#endif  // ASH_ASSISTANT_ASSISTANT_WEB_VIEW_DELEGATE_IMPL_H_

ash/assistant/ui/BUILD.gn

@@ -55,6 +55,7 @@ component("ui") {
     "assistant_web_container_view.h",
     "assistant_web_view.cc",
     "assistant_web_view.h",
+    "assistant_web_view_delegate.h",
     "base/assistant_button.cc",
     "base/assistant_button.h",
     "base/assistant_scroll_view.cc",

ash/assistant/ui/assistant_container_view.cc

@@ -332,16 +332,17 @@ void AssistantContainerView::Init() {
   layer()->SetFillsBoundsOpaquely(false);
 
   // Main view.
-  assistant_main_view_ = new AssistantMainViewDeprecated(delegate_);
-  AddChildView(assistant_main_view_);
+  assistant_main_view_ =
+      AddChildView(std::make_unique<AssistantMainViewDeprecated>(delegate_));
 
   // Mini view.
-  assistant_mini_view_ = new AssistantMiniView(delegate_);
-  AddChildView(assistant_mini_view_);
+  assistant_mini_view_ =
+      AddChildView(std::make_unique<AssistantMiniView>(delegate_));
 
   // Web view.
-  assistant_web_view_ = new AssistantWebView(delegate_);
-  AddChildView(assistant_web_view_);
+  assistant_web_view_ = AddChildView(std::make_unique<AssistantWebView>(
+      delegate_,
+      /*web_container_view_delegate=*/nullptr));
 
   // Update the view state based on the current UI mode.
   OnUiModeChanged(delegate_->GetUiModel()->ui_mode(),

ash/assistant/ui/assistant_web_container_view.cc

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <memory>
 
+#include "ash/assistant/model/assistant_ui_model.h"
 #include "ash/assistant/ui/assistant_ui_constants.h"
 #include "ash/assistant/ui/assistant_view_delegate.h"
 #include "ash/assistant/ui/assistant_web_view.h"
@@ -31,8 +32,10 @@ constexpr int kPreferredPaddingMinDip = 48;
 }  // namespace
 
 AssistantWebContainerView::AssistantWebContainerView(
-    AssistantViewDelegate* delegate)
-    : delegate_(delegate) {
+    AssistantViewDelegate* assistant_view_delegate,
+    AssistantWebViewDelegate* web_container_view_delegate)
+    : assistant_view_delegate_(assistant_view_delegate),
+      web_container_view_delegate_(web_container_view_delegate) {
   InitLayout();
 }
 
@@ -60,6 +63,14 @@ gfx::Size AssistantWebContainerView::CalculatePreferredSize() const {
   return gfx::Size(width, height - non_client_frame_view_height);
 }
 
+void AssistantWebContainerView::OnBackButtonPressed() {
+  assistant_web_view_->OnCaptionButtonPressed(AssistantButtonId::kBack);
+}
+
+views::View* AssistantWebContainerView::GetCaptionBarForTesting() {
+  return assistant_web_view_->caption_bar_for_testing();
+}
+
 void AssistantWebContainerView::InitLayout() {
   views::Widget::InitParams params;
   params.type = views::Widget::InitParams::TYPE_WINDOW;
@@ -73,7 +84,8 @@ void AssistantWebContainerView::InitLayout() {
   SetBackground(views::CreateSolidBackground(SK_ColorWHITE));
 
   // Web view.
-  AddChildView(std::make_unique<AssistantWebView>(delegate_));
+  assistant_web_view_ = AddChildView(std::make_unique<AssistantWebView>(
+      assistant_view_delegate_, web_container_view_delegate_));
 }
 
 }  // namespace ash

ash/assistant/ui/assistant_web_container_view.h

@@ -12,22 +12,34 @@
 namespace ash {
 
 class AssistantViewDelegate;
+class AssistantWebViewDelegate;
+class AssistantWebView;
 
 // The container of assistant_web_view when Assistant web container is enabled.
 class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebContainerView
     : public views::WidgetDelegateView {
  public:
-  explicit AssistantWebContainerView(AssistantViewDelegate* delegate);
+  AssistantWebContainerView(
+      AssistantViewDelegate* assistant_view_delegate,
+      AssistantWebViewDelegate* web_container_view_delegate);
   ~AssistantWebContainerView() override;
 
   // views::WidgetDelegateView:
   const char* GetClassName() const override;
   gfx::Size CalculatePreferredSize() const override;
 
+  void OnBackButtonPressed();
+
+  views::View* GetCaptionBarForTesting();
+
  private:
   void InitLayout();
 
-  AssistantViewDelegate* const delegate_;
+  AssistantViewDelegate* const assistant_view_delegate_;
+  AssistantWebViewDelegate* const web_container_view_delegate_;
+
+  // Owned by the views hierarchy.
+  AssistantWebView* assistant_web_view_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(AssistantWebContainerView);
 };

ash/assistant/ui/assistant_web_container_view_unittest.cc

@@ -11,8 +11,10 @@
 #include "ash/public/cpp/assistant/assistant_settings.h"
 #include "ash/shell.h"
 #include "base/macros.h"
+#include "base/run_loop.h"
 #include "base/test/scoped_feature_list.h"
 #include "chromeos/services/assistant/public/features.h"
+#include "ui/events/test/event_generator.h"
 
 namespace ash {
 
@@ -89,4 +91,28 @@ TEST_F(AssistantWebContainerViewTest, CenterWindow) {
   }
 }
 
+TEST_F(AssistantWebContainerViewTest, CloseWindowByKeyEvent) {
+  // Show Assistant Settings UI.
+  OpenAssistantSettings();
+  ASSERT_TRUE(view());
+
+  // Close Assistant Settings UI by key event.
+  ui::test::EventGenerator* generator = GetEventGenerator();
+  generator->PressKey(ui::VKEY_W, ui::EF_CONTROL_DOWN);
+  base::RunLoop().RunUntilIdle();
+  ASSERT_FALSE(view());
+}
+
+TEST_F(AssistantWebContainerViewTest, NoCaptionBarInAssistantWebView) {
+  // Show Assistant Settings UI.
+  OpenAssistantSettings();
+  AssistantWebContainerView* container_view = view();
+  ASSERT_TRUE(container_view);
+
+  // AssistantWebContainerView's widget should have its own caption buttons in
+  // the ash frame view. Therefore the AssistantWebView should not have the
+  // caption bar.
+  ASSERT_FALSE(container_view->GetCaptionBarForTesting());
+}
+
 }  // namespace ash

ash/assistant/ui/assistant_web_view.cc

@@ -9,8 +9,8 @@
 
 #include "ash/assistant/model/assistant_ui_model.h"
 #include "ash/assistant/ui/assistant_ui_constants.h"
-#include "ash/assistant/ui/assistant_view_delegate.h"
 #include "ash/assistant/ui/assistant_view_ids.h"
+#include "ash/assistant/ui/assistant_web_view_delegate.h"
 #include "ash/assistant/util/deep_link_util.h"
 #include "ash/public/cpp/app_list/app_list_features.h"
 #include "base/bind.h"
@@ -29,24 +29,27 @@ namespace ash {
 
 // AssistantWebView ------------------------------------------------------------
 
-AssistantWebView::AssistantWebView(AssistantViewDelegate* delegate)
-    : delegate_(delegate) {
+AssistantWebView::AssistantWebView(
+    AssistantViewDelegate* assistant_view_delegate,
+    AssistantWebViewDelegate* web_container_view_delegate)
+    : assistant_view_delegate_(assistant_view_delegate),
+      web_container_view_delegate_(web_container_view_delegate) {
   SetID(AssistantViewID::kWebView);
   InitLayout();
 
-  delegate_->AddObserver(this);
+  assistant_view_delegate_->AddObserver(this);
 
   // |AssistantWebView| has its own separate container when Assistant web
   // container is enabled. The container will handle its own lifecycle.
   if (!chromeos::assistant::features::IsAssistantWebContainerEnabled())
-    delegate_->AddUiModelObserver(this);
+    assistant_view_delegate_->AddUiModelObserver(this);
 }
 
 AssistantWebView::~AssistantWebView() {
   if (!chromeos::assistant::features::IsAssistantWebContainerEnabled())
-    delegate_->RemoveUiModelObserver(this);
+    assistant_view_delegate_->RemoveUiModelObserver(this);
 
-  delegate_->RemoveObserver(this);
+  assistant_view_delegate_->RemoveObserver(this);
 }
 
 const char* AssistantWebView::GetClassName() const {
@@ -68,7 +71,7 @@ int AssistantWebView::GetHeightForWidth(int width) const {
   // |height| <= |kMaxHeightDip|.
   // |height| should not exceed the height of the usable work area.
   const gfx::Rect usable_work_area =
-      delegate_->GetUiModel()->usable_work_area();
+      assistant_view_delegate_->GetUiModel()->usable_work_area();
 
   return std::min(kMaxHeightDip, usable_work_area.height());
 }
@@ -95,6 +98,10 @@ void AssistantWebView::InitLayout() {
   SetLayoutManager(std::make_unique<views::BoxLayout>(
       views::BoxLayout::Orientation::kVertical));
 
+  // Will use ash caption buttons when Assistant web container is enabled.
+  if (chromeos::assistant::features::IsAssistantWebContainerEnabled())
+    return;
+
   // Caption bar.
   caption_bar_ = new CaptionBar();
   caption_bar_->set_delegate(this);
@@ -114,7 +121,8 @@ bool AssistantWebView::OnCaptionButtonPressed(AssistantButtonId id) {
           // If we can't navigate back in the web contents' history stack we
           // defer back to our primary caption button delegate.
           if (!success && assistant_web_view) {
-            assistant_web_view->delegate_->GetCaptionBarDelegate()
+            assistant_web_view->assistant_view_delegate_
+                ->GetCaptionBarDelegate()
                 ->OnCaptionButtonPressed(AssistantButtonId::kBack);
           }
         },
@@ -123,7 +131,8 @@ bool AssistantWebView::OnCaptionButtonPressed(AssistantButtonId id) {
   }
 
   // For all other buttons we defer to our primary caption button delegate.
-  return delegate_->GetCaptionBarDelegate()->OnCaptionButtonPressed(id);
+  return assistant_view_delegate_->GetCaptionBarDelegate()
+      ->OnCaptionButtonPressed(id);
 }
 
 void AssistantWebView::OnDeepLinkReceived(
@@ -135,7 +144,7 @@ void AssistantWebView::OnDeepLinkReceived(
   RemoveContents();
 
   if (!contents_factory_.is_bound()) {
-    delegate_->GetNavigableContentsFactoryForView(
+    assistant_view_delegate_->GetNavigableContentsFactoryForView(
         contents_factory_.BindNewPipeAndPassReceiver());
   }
 
@@ -184,7 +193,7 @@ void AssistantWebView::DidSuppressNavigation(const GURL& url,
   // browser.
   if (assistant::util::IsDeepLinkUrl(url) ||
       disposition == WindowOpenDisposition::NEW_FOREGROUND_TAB) {
-    delegate_->OpenUrlFromView(url);
+    assistant_view_delegate_->OpenUrlFromView(url);
     return;
   }
 
@@ -192,6 +201,15 @@ void AssistantWebView::DidSuppressNavigation(const GURL& url,
   contents_->Navigate(url);
 }
 
+void AssistantWebView::UpdateCanGoBack(bool can_go_back) {
+  if (!chromeos::assistant::features::IsAssistantWebContainerEnabled())
+    return;
+
+  DCHECK(web_container_view_delegate_);
+  web_container_view_delegate_->UpdateBackButtonVisibility(GetWidget(),
+                                                           can_go_back);
+}
+
 void AssistantWebView::OnUiVisibilityChanged(
     AssistantVisibility new_visibility,
     AssistantVisibility old_visibility,
@@ -232,6 +250,11 @@ void AssistantWebView::UpdateContentSize() {
   if (!contents_ || !contents_view_initialized_)
     return;
 
+  if (chromeos::assistant::features::IsAssistantWebContainerEnabled()) {
+    contents_->GetView()->view()->SetPreferredSize(GetPreferredSize());
+    return;
+  }
+
   const gfx::Size preferred_size = gfx::Size(
       kPreferredWidthDip, GetHeightForWidth(kPreferredWidthDip) -
                               caption_bar_->GetPreferredSize().height());

ash/assistant/ui/assistant_web_view.h

@@ -22,7 +22,7 @@
 namespace ash {
 
 enum class AssistantButtonId;
-class AssistantViewDelegate;
+class AssistantWebViewDelegate;
 
 // AssistantWebView is a child of AssistantBubbleView which allows Assistant UI
 // to render remotely hosted content within its bubble. It provides a CaptionBar
@@ -35,7 +35,8 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
       public content::NavigableContentsObserver,
       public AssistantUiModelObserver {
  public:
-  explicit AssistantWebView(AssistantViewDelegate* delegate);
+  AssistantWebView(AssistantViewDelegate* assistant_view_delegate,
+                   AssistantWebViewDelegate* web_container_view_delegate);
   ~AssistantWebView() override;
 
   // views::View:
@@ -59,6 +60,7 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
   void DidSuppressNavigation(const GURL& url,
                              WindowOpenDisposition disposition,
                              bool from_user_gesture) override;
+  void UpdateCanGoBack(bool can_go_back) override;
 
   // AssistantUiModelObserver:
   void OnUiVisibilityChanged(
@@ -68,6 +70,8 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
       base::Optional<AssistantExitPoint> exit_point) override;
   void OnUsableWorkAreaChanged(const gfx::Rect& usable_work_area) override;
 
+  views::View* caption_bar_for_testing() { return caption_bar_; }
+
  private:
   void InitLayout();
   void RemoveContents();
@@ -77,9 +81,12 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
   // work area changed.
   void UpdateContentSize();
 
-  AssistantViewDelegate* const delegate_;
+  // TODO(b/143177141): Remove AssistantViewDelegate once standalone is
+  // deprecated.
+  AssistantViewDelegate* const assistant_view_delegate_;
+  AssistantWebViewDelegate* const web_container_view_delegate_;
 
-  CaptionBar* caption_bar_;  // Owned by view hierarchy.
+  CaptionBar* caption_bar_ = nullptr;  // Owned by view hierarchy.
 
   mojo::Remote<content::mojom::NavigableContentsFactory> contents_factory_;
   std::unique_ptr<content::NavigableContents> contents_;

ash/assistant/ui/assistant_web_view_delegate.h

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_ASSISTANT_UI_ASSISTANT_WEB_VIEW_DELEGATE_H_
+#define ASH_ASSISTANT_UI_ASSISTANT_WEB_VIEW_DELEGATE_H_
+
+#include "base/component_export.h"
+
+namespace views {
+class Widget;
+}  // namespace views
+
+namespace ash {
+
+// A delegate of web container views in assistant/ui.
+class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebViewDelegate {
+ public:
+  virtual ~AssistantWebViewDelegate() = default;
+
+  // Updates the visibility of the back button in Assistant web container.
+  virtual void UpdateBackButtonVisibility(views::Widget* widget,
+                                          bool visibility) = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_ASSISTANT_UI_ASSISTANT_WEB_VIEW_DELEGATE_H_

content/browser/content_service_delegate_impl.cc

@@ -131,6 +131,11 @@ class NavigableContentsDelegateImpl : public content::NavigableContentsDelegate,
     client_->DidAutoResizeView(new_size);
   }
 
+  void NavigationStateChanged(WebContents* source,
+                              InvalidateTypes changed_flags) override {
+    MaybeNotifyCanGoBack();
+  }
+
   // WebContentsObserver:
   void RenderViewReady() override {
     if (background_color_) {
@@ -175,6 +180,23 @@ class NavigableContentsDelegateImpl : public content::NavigableContentsDelegate,
 
   void DidStopLoading() override { client_->DidStopLoading(); }
 
+  void NavigationEntriesDeleted() override { MaybeNotifyCanGoBack(); }
+
+  void DidAttachInterstitialPage() override { MaybeNotifyCanGoBack(); }
+
+  void DidDetachInterstitialPage() override { MaybeNotifyCanGoBack(); }
+
+  // Notifies the client whether the web contents can navigate back in its
+  // history stack.
+  void MaybeNotifyCanGoBack() {
+    const bool can_go_back = web_contents_->GetController().CanGoBack();
+    if (can_go_back_ == can_go_back)
+      return;
+
+    can_go_back_ = can_go_back;
+    client_->UpdateCanGoBack(can_go_back);
+  }
+
   std::unique_ptr<WebContents> web_contents_;
   mojom::NavigableContentsClient* const client_;
 
@@ -183,6 +205,8 @@ class NavigableContentsDelegateImpl : public content::NavigableContentsDelegate,
   const gfx::Size auto_resize_max_size_;
   const base::Optional<SkColor> background_color_;
 
+  bool can_go_back_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(NavigableContentsDelegateImpl);
 };
 

services/content/public/cpp/navigable_contents.cc

@@ -95,6 +95,11 @@ void NavigableContents::DidSuppressNavigation(const GURL& url,
     observer.DidSuppressNavigation(url, disposition, from_user_gesture);
 }
 
+void NavigableContents::UpdateCanGoBack(bool can_go_back) {
+  for (auto& observer : observers_)
+    observer.UpdateCanGoBack(can_go_back);
+}
+
 void NavigableContents::UpdateContentAXTree(const ui::AXTreeID& id) {
   content_ax_tree_id_ = id;
   if (view_)

services/content/public/cpp/navigable_contents.h

@@ -83,6 +83,7 @@ class COMPONENT_EXPORT(CONTENT_SERVICE_CPP) NavigableContents
   void DidSuppressNavigation(const GURL& url,
                              WindowOpenDisposition disposition,
                              bool from_user_gesture) override;
+  void UpdateCanGoBack(bool can_go_back) override;
   void UpdateContentAXTree(const ui::AXTreeID& id) override;
 
   void OnEmbedTokenReceived(const base::UnguessableToken& token);

services/content/public/cpp/navigable_contents_observer.h

@@ -27,6 +27,7 @@ class COMPONENT_EXPORT(CONTENT_SERVICE_CPP) NavigableContentsObserver
   virtual void DidSuppressNavigation(const GURL& url,
                                      WindowOpenDisposition disposition,
                                      bool from_user_gesture) {}
+  virtual void UpdateCanGoBack(bool can_go_back) {}
 };
 
 }  // namespace content

services/content/public/mojom/navigable_contents.mojom

@@ -79,6 +79,10 @@ interface NavigableContentsClient {
                         ui.mojom.WindowOpenDisposition disposition,
                         bool from_user_gesture);
 
+  // Notifies the client whether the web contents can navigate back in its
+  // history stack.
+  UpdateCanGoBack(bool can_go_back);
+
   // Informs the client of the ID of the content area's accessibility tree
   // whenever it changes.
   UpdateContentAXTree(ax.mojom.AXTreeID id);

services/content/service_unittest.cc

@@ -43,6 +43,7 @@ class TestNavigableContentsClient : public mojom::NavigableContentsClient {
   void DidSuppressNavigation(const GURL& url,
                              WindowOpenDisposition disposition,
                              bool from_user_gesture) override {}
+  void UpdateCanGoBack(bool can_go_back) override {}
   void UpdateContentAXTree(const ui::AXTreeID& id) override {}
 
   DISALLOW_COPY_AND_ASSIGN(TestNavigableContentsClient);