Migrate Assistant Settings host off of Content Service.

Previously the host for Assistant Settings used the Content Service
for embedding WebContents, but this introduced a number for focus and
A11Y related bugs.

Now, we are migrating back to a WebView based implementation that
doesn't exhibit the same buggy behavior.

Manual tests run:
- Tab traverse in the forward direction
- Tab traverse in the backward direction
- ChromeVox traverse in the forward direction
- ChromeVox traverse in the backwards direction
- Changing between windows and then tab traversing
- Changing between windows and then ChromeVox traversing

Bug: b:146351046
Change-Id: I02601da8c8d726faaad527dda5785b4ccc60102b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999042Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#731770}

ash/BUILD.gn

@@ -2239,6 +2239,10 @@ static_library("test_support") {
     "assistant/test/assistant_ash_test_base.h",
     "assistant/test/test_assistant_service.cc",
     "assistant/test/test_assistant_service.h",
+    "assistant/test/test_assistant_web_view.cc",
+    "assistant/test/test_assistant_web_view.h",
+    "assistant/test/test_assistant_web_view_factory.cc",
+    "assistant/test/test_assistant_web_view_factory.h",
     "display/display_configuration_controller_test_api.cc",
     "display/display_configuration_controller_test_api.h",
     "display/mirror_window_test_api.cc",

ash/assistant/test/assistant_ash_test_base.cc

@@ -11,6 +11,7 @@
 #include "ash/app_list/views/assistant/assistant_main_view.h"
 #include "ash/app_list/views/assistant/assistant_page_view.h"
 #include "ash/assistant/assistant_controller.h"
+#include "ash/assistant/test/test_assistant_web_view_factory.h"
 #include "ash/keyboard/ui/keyboard_ui_controller.h"
 #include "ash/keyboard/ui/test/keyboard_test_util.h"
 #include "ash/public/cpp/app_list/app_list_features.h"
@@ -62,7 +63,8 @@ void PressHomeButton() {
 }  // namespace
 
 AssistantAshTestBase::AssistantAshTestBase()
-    : test_api_(AssistantTestApi::Create()) {}
+    : test_api_(AssistantTestApi::Create()),
+      test_web_view_factory_(std::make_unique<TestAssistantWebViewFactory>()) {}
 
 AssistantAshTestBase::~AssistantAshTestBase() = default;
 

ash/assistant/test/assistant_ash_test_base.h

@@ -23,8 +23,9 @@ namespace ash {
 
 class AssistantController;
 class AssistantInteractionController;
-class TestAssistantService;
 class AssistantTestApi;
+class TestAssistantService;
+class TestAssistantWebViewFactory;
 
 // Helper class to make testing the Assistant Ash UI easier.
 class AssistantAshTestBase : public AshTestBase {
@@ -145,6 +146,7 @@ class AssistantAshTestBase : public AshTestBase {
   TestAssistantService* assistant_service();
 
   std::unique_ptr<AssistantTestApi> test_api_;
+  std::unique_ptr<TestAssistantWebViewFactory> test_web_view_factory_;
   base::test::ScopedFeatureList scoped_feature_list_;
   AssistantController* controller_ = nullptr;
 

ash/assistant/test/test_assistant_web_view.cc

+// Copyright 2020 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/test/test_assistant_web_view.h"
+
+#include "base/threading/sequenced_task_runner_handle.h"
+
+namespace ash {
+
+TestAssistantWebView::TestAssistantWebView() = default;
+
+TestAssistantWebView::~TestAssistantWebView() = default;
+
+void TestAssistantWebView::AddObserver(Observer* observer) {
+  observers_.AddObserver(observer);
+}
+
+void TestAssistantWebView::RemoveObserver(Observer* observer) {
+  observers_.RemoveObserver(observer);
+}
+
+gfx::NativeView TestAssistantWebView::GetNativeView() {
+  // Not yet implemented for unittests.
+  return nullptr;
+}
+
+bool TestAssistantWebView::GoBack() {
+  // Not yet implemented for unittests.
+  return false;
+}
+
+void TestAssistantWebView::Navigate(const GURL& url) {
+  // Simulate navigation by notifying |observers_| of the expected event that
+  // would normally signal navigation completion. We do this asynchronously to
+  // more accurately simulate real-world conditions.
+  base::SequencedTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE, base::BindOnce(
+                     [](const base::WeakPtr<TestAssistantWebView>& self) {
+                       if (self) {
+                         for (auto& observer : self->observers_)
+                           observer.DidStopLoading();
+                       }
+                     },
+                     weak_factory_.GetWeakPtr()));
+}
+
+}  // namespace ash

ash/assistant/test/test_assistant_web_view.h

+// Copyright 2020 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_TEST_TEST_ASSISTANT_WEB_VIEW_H_
+#define ASH_ASSISTANT_TEST_TEST_ASSISTANT_WEB_VIEW_H_
+
+#include "ash/public/cpp/assistant/assistant_web_view_2.h"
+#include "base/observer_list.h"
+
+namespace ash {
+
+// An implementation of AssistantWebView2 for use in unittests.
+class TestAssistantWebView : public AssistantWebView2 {
+ public:
+  TestAssistantWebView();
+  TestAssistantWebView(const TestAssistantWebView& copy) = delete;
+  TestAssistantWebView& operator=(const TestAssistantWebView& assign) = delete;
+  ~TestAssistantWebView() override;
+
+  // AssistantWebView2:
+  void AddObserver(Observer* observer) override;
+  void RemoveObserver(Observer* observer) override;
+  gfx::NativeView GetNativeView() override;
+  bool GoBack() override;
+  void Navigate(const GURL& url) override;
+
+ private:
+  base::ObserverList<Observer> observers_;
+
+  base::WeakPtrFactory<TestAssistantWebView> weak_factory_{this};
+};
+
+}  // namespace ash
+
+#endif  // ASH_ASSISTANT_TEST_TEST_ASSISTANT_WEB_VIEW_H_

ash/assistant/test/test_assistant_web_view_factory.cc

+// Copyright 2020 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/test/test_assistant_web_view_factory.h"
+
+#include "ash/assistant/test/test_assistant_web_view.h"
+
+namespace ash {
+
+TestAssistantWebViewFactory::TestAssistantWebViewFactory() = default;
+
+TestAssistantWebViewFactory::~TestAssistantWebViewFactory() = default;
+
+std::unique_ptr<AssistantWebView2> TestAssistantWebViewFactory::Create(
+    const AssistantWebView2::InitParams& params) {
+  return std::make_unique<TestAssistantWebView>();
+}
+
+}  // namespace ash
\ No newline at end of file

ash/assistant/test/test_assistant_web_view_factory.h

+// Copyright 2020 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_TEST_TEST_ASSISTANT_WEB_VIEW_FACTORY_H_
+#define ASH_ASSISTANT_TEST_TEST_ASSISTANT_WEB_VIEW_FACTORY_H_
+
+#include <memory>
+
+#include "ash/public/cpp/assistant/assistant_web_view_factory.h"
+
+namespace ash {
+
+// An implementation of AssistantWebViewFactory for use in unittests.
+class TestAssistantWebViewFactory : public AssistantWebViewFactory {
+ public:
+  TestAssistantWebViewFactory();
+  TestAssistantWebViewFactory(const AssistantWebViewFactory& copy) = delete;
+  TestAssistantWebViewFactory& operator=(
+      const AssistantWebViewFactory& assign) = delete;
+  ~TestAssistantWebViewFactory() override;
+
+  // AssistantWebViewFactory:
+  std::unique_ptr<AssistantWebView2> Create(
+      const AssistantWebView2::InitParams& params) override;
+};
+
+}  // namespace ash
+
+#endif  // ASH_ASSISTANT_TEST_TEST_ASSISTANT_WEB_VIEW_FACTORY_H_

ash/assistant/ui/assistant_web_container_view.cc

@@ -85,12 +85,6 @@ void AssistantWebContainerView::InitLayout() {
   views::Widget* widget = new views::Widget;
   widget->Init(std::move(params));
 
-  // TODO(b/146351046): Temporary workaround for an a11y bug b/144765770.
-  // Should be removed once we have moved off of the Content Service
-  // (tracked in b/146351046).
-  widget->client_view()->SetFocusBehavior(FocusBehavior::ALWAYS);
-  widget->client_view()->RequestFocus();
-
   SetLayoutManager(std::make_unique<views::FillLayout>());
   SetBackground(views::CreateSolidBackground(SK_ColorWHITE));
 

ash/assistant/ui/assistant_web_view.cc

@@ -13,10 +13,10 @@
 #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 "ash/public/cpp/assistant/assistant_web_view_factory.h"
 #include "base/bind.h"
 #include "base/callback.h"
 #include "chromeos/services/assistant/public/features.h"
-#include "services/content/public/cpp/navigable_contents_view.h"
 #include "ui/aura/window.h"
 #include "ui/compositor/layer.h"
 #include "ui/display/display.h"
@@ -80,27 +80,6 @@ void AssistantWebView::ChildPreferredSizeChanged(views::View* child) {
   SchedulePaint();
 }
 
-void AssistantWebView::OnFocus() {
-  if (contents_)
-    contents_->Focus();
-}
-
-void AssistantWebView::AboutToRequestFocusFromTabTraversal(bool reverse) {
-  if (contents_) {
-    // TODO(b/146351046): Temporary workaround for b/145213680. Should be
-    // removed once we have moved off of the Content Service (tracked in
-    // b/146351046).
-    base::SequencedTaskRunnerHandle::Get()->PostTask(
-        FROM_HERE, base::BindOnce(
-                       [](base::WeakPtr<AssistantWebView> view) {
-                         if (view)
-                           view->GetFocusManager()->ClearFocus();
-                       },
-                       weak_factory_.GetWeakPtr()));
-    contents_->FocusThroughTabTraversal(reverse);
-  }
-}
-
 void AssistantWebView::InitLayout() {
   SetLayoutManager(std::make_unique<views::BoxLayout>(
       views::BoxLayout::Orientation::kVertical));
@@ -120,20 +99,10 @@ void AssistantWebView::InitLayout() {
 
 bool AssistantWebView::OnCaptionButtonPressed(AssistantButtonId id) {
   // We need special handling of the back button. When possible, the back button
-  // should navigate backwards in the web contents' history stack.
-  if (id == AssistantButtonId::kBack && contents_) {
-    contents_->GoBack(base::BindOnce(
-        [](const base::WeakPtr<AssistantWebView>& assistant_web_view,
-           bool success) {
-          // 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->assistant_view_delegate_
-                ->GetCaptionBarDelegate()
-                ->OnCaptionButtonPressed(AssistantButtonId::kBack);
-          }
-        },
-        weak_factory_.GetWeakPtr()));
+  // should navigate backwards in the WebContents' history stack. If we can't go
+  // back, control is returned to the primary caption button delegate.
+  if (id == AssistantButtonId::kBack && contents_view_ &&
+      contents_view_->GoBack()) {
     return true;
   }
 
@@ -144,20 +113,20 @@ bool AssistantWebView::OnCaptionButtonPressed(AssistantButtonId id) {
 
 void AssistantWebView::DidStopLoading() {
   // We should only respond to the |DidStopLoading| event the first time, to add
-  // the view for the navigable contents to our view hierarchy and perform other
-  // one-time view initializations.
+  // the view for contents to our view hierarchy and perform other one-time view
+  // initializations.
   if (contents_view_initialized_)
     return;
 
   contents_view_initialized_ = true;
 
   UpdateContentSize();
-  AddChildView(contents_->GetView()->view());
+  AddChildView(contents_view_.get());
   SetFocusBehavior(FocusBehavior::ALWAYS);
 
-  // We need to clip the corners of our web contents to match our container.
+  // We need to clip the corners of our WebContents to match our container.
   if (!chromeos::assistant::features::IsAssistantWebContainerEnabled()) {
-    contents_->GetView()->native_view()->layer()->SetRoundedCornerRadius(
+    contents_view_->GetNativeView()->layer()->SetRoundedCornerRadius(
         {/*top_left=*/0, /*top_right=*/0, /*bottom_right=*/kCornerRadiusDip,
          /*bottom_left=*/kCornerRadiusDip});
   }
@@ -179,11 +148,11 @@ void AssistantWebView::DidSuppressNavigation(const GURL& url,
     return;
   }
 
-  // Otherwise we'll allow our web contents to navigate freely.
-  contents_->Navigate(url);
+  // Otherwise we'll allow our WebContents to navigate freely.
+  contents_view_->Navigate(url);
 }
 
-void AssistantWebView::UpdateCanGoBack(bool can_go_back) {
+void AssistantWebView::DidChangeCanGoBack(bool can_go_back) {
   if (!chromeos::assistant::features::IsAssistantWebContainerEnabled())
     return;
 
@@ -210,23 +179,22 @@ void AssistantWebView::OnUiVisibilityChanged(
 void AssistantWebView::OpenUrl(const GURL& url) {
   RemoveContents();
 
-  if (!contents_factory_.is_bound()) {
-    assistant_view_delegate_->GetNavigableContentsFactoryForView(
-        contents_factory_.BindNewPipeAndPassReceiver());
-  }
+  AssistantWebView2::InitParams contents_params;
+  contents_params.suppress_navigation = true;
 
-  auto contents_params = content::mojom::NavigableContentsParams::New();
-  contents_params->suppress_navigations = true;
+  contents_view_ = AssistantWebViewFactory::Get()->Create(contents_params);
 
-  contents_ = std::make_unique<content::NavigableContents>(
-      contents_factory_.get(), std::move(contents_params));
+  // We retain ownership of |contents_view_| as it is only added to the view
+  // hierarchy once loading stops and we want to ensure that it is cleaned up in
+  // the rare chance that that never occurs.
+  contents_view_->set_owned_by_client();
 
-  // We observe |contents_| so that we can handle events from the underlying
-  // web contents.
-  contents_->AddObserver(this);
+  // We observe |contents_view_| so that we can handle events from the
+  // underlying WebContents.
+  contents_view_->AddObserver(this);
 
   // Navigate to the specified |url|.
-  contents_->Navigate(url);
+  contents_view_->Navigate(url);
 }
 
 void AssistantWebView::OnUsableWorkAreaChanged(
@@ -237,32 +205,32 @@ void AssistantWebView::OnUsableWorkAreaChanged(
 }
 
 void AssistantWebView::RemoveContents() {
-  if (!contents_)
+  if (!contents_view_)
     return;
 
-  views::View* view = contents_->GetView()->view();
-  if (view)
-    RemoveChildView(view);
+  RemoveChildView(contents_view_.get());
 
   SetFocusBehavior(FocusBehavior::NEVER);
-  contents_->RemoveObserver(this);
-  contents_.reset();
+
+  contents_view_->RemoveObserver(this);
+  contents_view_.reset();
+
   contents_view_initialized_ = false;
 }
 
 void AssistantWebView::UpdateContentSize() {
-  if (!contents_ || !contents_view_initialized_)
+  if (!contents_view_ || !contents_view_initialized_)
     return;
 
   if (chromeos::assistant::features::IsAssistantWebContainerEnabled()) {
-    contents_->GetView()->view()->SetPreferredSize(GetPreferredSize());
+    contents_view_->SetPreferredSize(GetPreferredSize());
     return;
   }
 
   const gfx::Size preferred_size = gfx::Size(
       kPreferredWidthDip, GetHeightForWidth(kPreferredWidthDip) -
                               caption_bar_->GetPreferredSize().height());
-  contents_->GetView()->view()->SetPreferredSize(preferred_size);
+  contents_view_->SetPreferredSize(preferred_size);
 }
 
 }  // namespace ash

ash/assistant/ui/assistant_web_view.h

@@ -12,11 +12,10 @@
 #include "ash/assistant/model/assistant_ui_model_observer.h"
 #include "ash/assistant/ui/assistant_view_delegate.h"
 #include "ash/assistant/ui/caption_bar.h"
+#include "ash/public/cpp/assistant/assistant_web_view_2.h"
 #include "base/component_export.h"
 #include "base/macros.h"
 #include "base/optional.h"
-#include "mojo/public/cpp/bindings/remote.h"
-#include "services/content/public/cpp/navigable_contents.h"
 #include "ui/views/view.h"
 
 namespace ash {
@@ -28,12 +27,11 @@ class AssistantWebViewDelegate;
 // standalone Assistant UI.
 // AssistantWebView is a child of AssistantContainerView which allows Assistant
 // UI to render remotely hosted content within its bubble. It provides a
-// CaptionBar for window level controls and embeds web contents with help from
-// the Content Service.
+// CaptionBar for window level controls and embeds WebContents.
 class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
     : public views::View,
       public CaptionBarDelegate,
-      public content::NavigableContentsObserver,
+      public AssistantWebView2::Observer,
       public AssistantUiModelObserver {
  public:
   AssistantWebView(AssistantViewDelegate* assistant_view_delegate,
@@ -45,18 +43,16 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
   gfx::Size CalculatePreferredSize() const override;
   int GetHeightForWidth(int width) const override;
   void ChildPreferredSizeChanged(views::View* child) override;
-  void OnFocus() override;
-  void AboutToRequestFocusFromTabTraversal(bool reverse) override;
 
   // CaptionBarDelegate:
   bool OnCaptionButtonPressed(AssistantButtonId id) override;
 
-  // content::NavigableContentsObserver:
+  // AssistantWebView2::Observer:
   void DidStopLoading() override;
   void DidSuppressNavigation(const GURL& url,
                              WindowOpenDisposition disposition,
                              bool from_user_gesture) override;
-  void UpdateCanGoBack(bool can_go_back) override;
+  void DidChangeCanGoBack(bool can_go_back) override;
 
   // AssistantUiModelObserver:
   void OnUiVisibilityChanged(
@@ -86,14 +82,10 @@ class COMPONENT_EXPORT(ASSISTANT_UI) AssistantWebView
   AssistantWebViewDelegate* const web_container_view_delegate_;
 
   CaptionBar* caption_bar_ = nullptr;  // Owned by view hierarchy.
-
-  mojo::Remote<content::mojom::NavigableContentsFactory> contents_factory_;
-  std::unique_ptr<content::NavigableContents> contents_;
+  std::unique_ptr<AssistantWebView2> contents_view_;
 
   bool contents_view_initialized_ = false;
 
-  base::WeakPtrFactory<AssistantWebView> weak_factory_{this};
-
   DISALLOW_COPY_AND_ASSIGN(AssistantWebView);
 };
 

ash/public/cpp/assistant/assistant_web_view_2.h

@@ -17,8 +17,8 @@ namespace ash {
 
 // TODO(b/146520500): Rename to AssistantWebView after freeing up name which is
 // currently in use.
-// A view which wraps a views::WebView (and associated content::WebContents) to
-// work around dependency restrictions in Ash.
+// A view which wraps a views::WebView (and associated WebContents) to work
+// around dependency restrictions in Ash.
 class ASH_PUBLIC_EXPORT AssistantWebView2 : public views::View {
  public:
   // Initialization parameters which dictate how an instance of
@@ -29,14 +29,14 @@ class ASH_PUBLIC_EXPORT AssistantWebView2 : public views::View {
     ~InitParams();
 
     // If enabled, AssistantWebView2 will automatically resize to the size
-    // desired by its embedded content::WebContents. Note that, if specified,
-    // the content::WebContents will be bounded by |min_size| and |max_size|.
+    // desired by its embedded WebContents. Note that, if specified, the
+    // WebContents will be bounded by |min_size| and |max_size|.
     bool enable_auto_resize = false;
     base::Optional<gfx::Size> min_size = base::nullopt;
     base::Optional<gfx::Size> max_size = base::nullopt;
 
     // If enabled, AssistantWebView2 will suppress navigation attempts of its
-    // embedded content::WebContents. When navigation suppression occurs,
+    // embedded WebContents. When navigation suppression occurs,
     // Observer::DidSuppressNavigation() will be invoked.
     bool suppress_navigation = false;
   };
@@ -44,16 +44,19 @@ class ASH_PUBLIC_EXPORT AssistantWebView2 : public views::View {
   // An observer which receives AssistantWebView2 events.
   class Observer : public base::CheckedObserver {
    public:
-    // Invoked when the embedded content::WebContents has stopped loading.
+    // Invoked when the embedded WebContents has stopped loading.
     virtual void DidStopLoading() {}
 
-    // Invoked when the embedded content::WebContents has suppressed navigation.
+    // Invoked when the embedded WebContents has suppressed navigation.
     virtual void DidSuppressNavigation(const GURL& url,
                                        WindowOpenDisposition disposition,
                                        bool from_user_gesture) {}
 
-    // Invoked when the focused node within the embedded content::WebContents
-    // has changed.
+    // Invoked when the embedded WebContents' ability to go back has changed.
+    virtual void DidChangeCanGoBack(bool can_go_back) {}
+
+    // Invoked when the focused node within the embedded WebContents has
+    // changed.
     virtual void DidChangeFocusedNode(const gfx::Rect& node_bounds_in_screen) {}
   };
 
@@ -63,12 +66,15 @@ class ASH_PUBLIC_EXPORT AssistantWebView2 : public views::View {
   virtual void AddObserver(Observer* observer) = 0;
   virtual void RemoveObserver(Observer* observer) = 0;
 
-  // Invoke to navigate back in the embedded content::WebContents' navigation
-  // stack. If backwards navigation is not possible, return |false|. Otherwise
-  // returns |true| to indicate success.
+  // Returns the native view associated w/ the underlying WebContents.
+  virtual gfx::NativeView GetNativeView() = 0;
+
+  // Invoke to navigate back in the embedded WebContents' navigation stack. If
+  // backwards navigation is not possible, return |false|. Otherwise returns
+  // |true| to indicate success.
   virtual bool GoBack() = 0;
 
-  // Invoke to navigate the embedded content::WebContents' to |url|.
+  // Invoke to navigate the embedded WebContents' to |url|.
   virtual void Navigate(const GURL& url) = 0;
 
  protected:

chrome/browser/ui/ash/assistant/assistant_web_view_impl.cc

@@ -31,6 +31,10 @@ const char* AssistantWebViewImpl::GetClassName() const {
   return "AssistantWebViewImpl";
 }
 
+gfx::NativeView AssistantWebViewImpl::GetNativeView() {
+  return web_contents_->GetNativeView();
+}
+
 void AssistantWebViewImpl::ChildPreferredSizeChanged(views::View* child) {
   DCHECK_EQ(web_view_.get(), child);
   SetPreferredSize(web_view_->GetPreferredSize());
@@ -109,6 +113,13 @@ bool AssistantWebViewImpl::TakeFocus(content::WebContents* web_contents,
   return false;
 }
 
+void AssistantWebViewImpl::NavigationStateChanged(
+    content::WebContents* web_contents,
+    content::InvalidateTypes changed_flags) {
+  DCHECK_EQ(web_contents_.get(), web_contents);
+  UpdateCanGoBack();
+}
+
 void AssistantWebViewImpl::DidStopLoading() {
   for (auto& observer : observers_)
     observer.DidStopLoading();
@@ -141,6 +152,18 @@ void AssistantWebViewImpl::RenderViewHostChanged(
                                                              max_size);
 }
 
+void AssistantWebViewImpl::NavigationEntriesDeleted() {
+  UpdateCanGoBack();
+}
+
+void AssistantWebViewImpl::DidAttachInterstitialPage() {
+  UpdateCanGoBack();
+}
+
+void AssistantWebViewImpl::DidDetachInterstitialPage() {
+  UpdateCanGoBack();
+}
+
 void AssistantWebViewImpl::InitWebContents(Profile* profile) {
   web_contents_ =
       content::WebContents::Create(content::WebContents::CreateParams(
@@ -168,3 +191,14 @@ void AssistantWebViewImpl::InitLayout(Profile* profile) {
   web_view_->SetWebContents(web_contents_.get());
   AddChildView(web_view_.get());
 }
+
+void AssistantWebViewImpl::UpdateCanGoBack() {
+  const bool can_go_back = web_contents_->GetController().CanGoBack();
+  if (can_go_back_ == can_go_back)
+    return;
+
+  can_go_back_ = can_go_back;
+
+  for (auto& observer : observers_)
+    observer.DidChangeCanGoBack(can_go_back_);
+}

chrome/browser/ui/ash/assistant/assistant_web_view_impl.h

@@ -33,6 +33,7 @@ class AssistantWebViewImpl : public ash::AssistantWebView2,
 
   // ash::AssistantWebView2:
   const char* GetClassName() const override;
+  gfx::NativeView GetNativeView() override;
   void ChildPreferredSizeChanged(views::View* child) override;
   void Layout() override;
   void AddObserver(Observer* observer) override;
@@ -53,22 +54,32 @@ class AssistantWebViewImpl : public ash::AssistantWebView2,
   void ResizeDueToAutoResize(content::WebContents* web_contents,
                              const gfx::Size& new_size) override;
   bool TakeFocus(content::WebContents* web_contents, bool reverse) override;
+  void NavigationStateChanged(content::WebContents* web_contents,
+                              content::InvalidateTypes changed_flags) override;
 
   // content::WebContentsObserver:
   void DidStopLoading() override;
   void OnFocusChangedInPage(content::FocusedNodeDetails* details) override;
   void RenderViewHostChanged(content::RenderViewHost* old_host,
                              content::RenderViewHost* new_host) override;
+  void NavigationEntriesDeleted() override;
+  void DidAttachInterstitialPage() override;
+  void DidDetachInterstitialPage() override;
 
  private:
   void InitWebContents(Profile* profile);
   void InitLayout(Profile* profile);
 
+  void UpdateCanGoBack();
+
   const InitParams params_;
 
   std::unique_ptr<content::WebContents> web_contents_;
   std::unique_ptr<views::WebView> web_view_;
 
+  // Whether or not the embedded |web_contents_| can go back.
+  bool can_go_back_ = false;
+
   base::ObserverList<Observer> observers_;
 };