assistant: add shelf layout unit tests

* Add a couple shelf layout tests to make sure when Assistant
  is invoked in tablet mode with/without apps, the hotseat state
  won't change.

Bug: None
Test: unit tests
Change-Id: I7103fe9489f780cd441942801aa4d03b71825dbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013752
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734728}

ash/app_list/views/assistant/assistant_test_api_impl.cc

@@ -133,6 +133,10 @@ AssistantState* AssistantTestApiImpl::GetAssistantState() {
   return AssistantState::Get();
 }
 
+void AssistantTestApiImpl::WaitUntilIdle() {
+  base::RunLoop().RunUntilIdle();
+}
+
 void AssistantTestApiImpl::EnableAnimations() {
   scoped_animation_duration_ = nullptr;
   AppListView::SetShortAnimationForTesting(false);

ash/app_list/views/assistant/assistant_test_api_impl.h

@@ -33,6 +33,7 @@ class AssistantTestApiImpl : public AssistantTestApi {
   void SetTabletMode(bool enable) override;
   void SetPreferVoice(bool value) override;
   AssistantState* GetAssistantState() override;
+  void WaitUntilIdle() override;
   views::View* page_view() override;
   views::View* main_view() override;
   views::View* ui_element_container() override;

ash/public/cpp/test/assistant_test_api.h

@@ -55,6 +55,9 @@ class ASH_EXPORT AssistantTestApi {
   // Returns the interface to get/set Assistant related prefs and states.
   virtual AssistantState* GetAssistantState() = 0;
 
+  // Wait for all Assistant related actions to settle.
+  virtual void WaitUntilIdle() = 0;
+
   // Returns the top-level Assistant specific view.
   // Can only be used after the Assistant UI has been shown at least once.
   virtual views::View* page_view() = 0;

ash/shelf/test/shelf_layout_manager_test_base.h

@@ -5,6 +5,8 @@
 #ifndef ASH_SHELF_TEST_SHELF_LAYOUT_MANAGER_TEST_BASE_H_
 #define ASH_SHELF_TEST_SHELF_LAYOUT_MANAGER_TEST_BASE_H_
 
+#include <utility>
+
 #include "ash/public/cpp/shelf_types.h"
 #include "ash/test/ash_test_base.h"
 #include "ash/wm/workspace/workspace_types.h"
@@ -19,7 +21,9 @@ class ShelfLayoutManager;
 
 class ShelfLayoutManagerTestBase : public AshTestBase {
  public:
-  ShelfLayoutManagerTestBase() = default;
+  template <typename... TaskEnvironmentTraits>
+  explicit ShelfLayoutManagerTestBase(TaskEnvironmentTraits&&... traits)
+      : AshTestBase(std::forward<TaskEnvironmentTraits>(traits)...) {}
 
   // Calls the private SetState() function.
   void SetState(ShelfLayoutManager* layout_manager, ShelfVisibilityState state);