Reland "Fix crash when exiting mirror mode on tablet"

This is a reland of b2af1539 which was reverted
due to being pointed out by SoM as a cause of an Android compile failure.

Original change's description:
> Fix crash when exiting mirror mode on tablet
>
> Attempting to create ShelfWidget while shelf is in-app is causing
> a crash because ShelfWidget::DelegateView::UpdateOpaqueBackground is
> called before the widget is initialized.
>
> Moving that call from ShelfWidget::DelegateView constructor into
> ShelfWidget::DelegateView::OnWidgetInitialized.
>
> Added a unit test.
>
> Bug: 1021662
> Test: manual, DisplayManagerTest.ExitMirrorModeInTabletMode
> Change-Id: I32a5f91c834d673b109ab518bc2600908d8dea5c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902273
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Manu Cornet <manucornet@chromium.org>
> Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#713464}

TBR=oshima@chromium.org

Bug: 1021662
Change-Id: If28233c15d870fca0c9f6818e9031d98276bfe16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904159
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713547}

ash/display/display_manager_unittest.cc

@@ -4612,4 +4612,24 @@ TEST_F(DisplayManagerTest, UpdateRootWindowForNewWindows) {
   }
 }
 
+// Test that exiting mirror mode in tablet mode with a fullscreen window
+// does not cause a crash (crbug.com/1021662).
+TEST_F(DisplayManagerTest, ExitMirrorModeInTabletMode) {
+  // Simulate a tablet with and external display connected.
+  display::test::DisplayManagerTestApi(display_manager())
+      .SetFirstDisplayAsInternalDisplay();
+  UpdateDisplay("800x600,800x600");
+  Shell::Get()->tablet_mode_controller()->SetEnabledForTest(true);
+  base::RunLoop().RunUntilIdle();
+  EXPECT_TRUE(display_manager()->IsInSoftwareMirrorMode());
+
+  // Create a window to force in-app shelf.
+  std::unique_ptr<aura::Window> window = CreateTestWindow();
+
+  // Exit mirror mode.
+  display_manager()->SetMirrorMode(display::MirrorMode::kOff, base::nullopt);
+  base::RunLoop().RunUntilIdle();
+  EXPECT_FALSE(display_manager()->IsInSoftwareMirrorMode());
+}
+
 }  // namespace ash

ash/shelf/shelf_widget.cc

@@ -108,6 +108,8 @@ class ShelfWidget::DelegateView : public views::WidgetDelegate,
 
   bool CanActivate() const override;
   void ReorderChildLayers(ui::Layer* parent_layer) override;
+  void OnWidgetInitialized() override;
+
   void UpdateBackgroundBlur();
   void UpdateOpaqueBackground();
   void UpdateDragHandle();
@@ -169,8 +171,6 @@ ShelfWidget::DelegateView::DelegateView(ShelfWidget* shelf_widget)
   drag_handle_->layer()->SetRoundedCornerRadius(
       {radius, radius, radius, radius});
   drag_handle_->SetSize(kDragHandleSize);
-
-  UpdateOpaqueBackground();
 }
 
 ShelfWidget::DelegateView::~DelegateView() = default;
@@ -211,6 +211,10 @@ void ShelfWidget::DelegateView::ReorderChildLayers(ui::Layer* parent_layer) {
   parent_layer->StackAtBottom(&opaque_background_);
 }
 
+void ShelfWidget::DelegateView::OnWidgetInitialized() {
+  UpdateOpaqueBackground();
+}
+
 void ShelfWidget::DelegateView::UpdateBackgroundBlur() {
   // Blur only if the background is visible.
   const bool should_blur_background =