Revert of Replaced BackgroundAnimator with ShelfBackgroundAnimator.

ash/common/system/status_area_widget.h

Index: ash/common/system/status_area_widget.h
diff --git a/ash/common/system/status_area_widget.h b/ash/common/system/status_area_widget.h
index 562949612f0d98903f2ac0f73c4e0a6495a4c91b..414a7f5e1046a159a08a8f33dfd1d61e3066597b 100644
--- a/ash/common/system/status_area_widget.h
+++ b/ash/common/system/status_area_widget.h
@@ -7,7 +7,6 @@
 
 #include "ash/ash_export.h"
 #include "ash/common/login_status.h"
-#include "ash/common/shelf/shelf_background_animator_observer.h"
 #include "ash/common/shelf/shelf_types.h"
 #include "base/macros.h"
 #include "ui/views/widget/widget.h"
@@ -27,8 +26,7 @@
 class VirtualKeyboardTray;
 #endif
 
-class ASH_EXPORT StatusAreaWidget : public views::Widget,
-                                    public ShelfBackgroundAnimatorObserver {
+class ASH_EXPORT StatusAreaWidget : public views::Widget {
  public:
   StatusAreaWidget(WmWindow* status_container, WmShelf* wm_shelf);
   ~StatusAreaWidget() override;
@@ -82,9 +80,6 @@
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
 
-  // ShelfBackgroundAnimatorObserver:
-  void UpdateShelfItemBackground(int alpha) override;
-
  private:
   void AddSystemTray();
   void AddWebNotificationTray();