Reland of "Apply Shelf/Dock opacity animations for the Material Design style."

ash/common/shelf/app_list_button.h

Index: ash/common/shelf/app_list_button.h
diff --git a/ash/common/shelf/app_list_button.h b/ash/common/shelf/app_list_button.h
index 0fabb4a62db82825639115978892a029acdb596a..af0fdbd9d4192fd552694607b66fa31ae5b5ab5f 100644
--- a/ash/common/shelf/app_list_button.h
+++ b/ash/common/shelf/app_list_button.h
@@ -27,6 +27,9 @@ class ASH_EXPORT AppListButton : public views::ImageButton {
 
   bool draw_background_as_active() { return draw_background_as_active_; }
 
+  // Sets alpha value of the background and schedules a paint.
+  void SetBackgroundAlpha(int alpha);
+
  protected:
   // views::ImageButton overrides:
   bool OnMousePressed(const ui::MouseEvent& event) override;
@@ -61,6 +64,9 @@ class ASH_EXPORT AppListButton : public views::ImageButton {
   // the application list.
   bool draw_background_as_active_;
 
+  // Alpha value used to paint the background.
+  int background_alpha_;
+
   InkDropButtonListener* listener_;
   ShelfView* shelf_view_;
   WmShelf* wm_shelf_;