Replaced BackgroundAnimator with ShelfBackgroundAnimator.

ash/common/shelf/overflow_button.h

Index: ash/common/shelf/overflow_button.h
diff --git a/ash/common/shelf/overflow_button.h b/ash/common/shelf/overflow_button.h
index ac2a4faf2b65a1fa037eed75d7426d36c92500a6..b96479fbf4f856f10188d1758d54ca17d2699680 100644
--- a/ash/common/shelf/overflow_button.h
+++ b/ash/common/shelf/overflow_button.h
@@ -21,6 +21,9 @@ class OverflowButton : public views::CustomButton {
 
   void OnShelfAlignmentChanged();
 
+  // Sets alpha value of the background and schedules a paint.
+  void SetBackgroundAlpha(int alpha);
+
  private:
   // views::View:
   void OnPaint(gfx::Canvas* canvas) override;
@@ -58,6 +61,9 @@ class OverflowButton : public views::CustomButton {
   InkDropButtonListener* listener_;
   WmShelf* wm_shelf_;
 
+  // Alpha value used to paint the background.
+  int background_alpha_;
+
   DISALLOW_COPY_AND_ASSIGN(OverflowButton);
 };