Replaced BackgroundAnimator with ShelfBackgroundAnimator.

ash/system/status_area_widget.h

Index: ash/system/status_area_widget.h
diff --git a/ash/system/status_area_widget.h b/ash/system/status_area_widget.h
index cb492418b03d000744461df5325ee52befdf62b7..294673a87a476c06d737efe856cd1cd83851c8b5 100644
--- a/ash/system/status_area_widget.h
+++ b/ash/system/status_area_widget.h
@@ -6,6 +6,7 @@
 #define ASH_SYSTEM_STATUS_AREA_WIDGET_H_
 
 #include "ash/ash_export.h"
+#include "ash/common/shelf/shelf_background_animator_observer.h"
 #include "ash/common/shelf/shelf_types.h"
 #include "ash/system/user/login_status.h"
 #include "base/macros.h"
@@ -24,7 +25,8 @@ class LogoutButtonTray;
 class VirtualKeyboardTray;
 #endif
 
-class ASH_EXPORT StatusAreaWidget : public views::Widget {
+class ASH_EXPORT StatusAreaWidget : public views::Widget,
+                                    public ShelfBackgroundAnimatorObserver {
  public:
   StatusAreaWidget(WmWindow* status_container, ShelfWidget* shelf_widget);
   ~StatusAreaWidget() override;
@@ -78,6 +80,9 @@ class ASH_EXPORT StatusAreaWidget : public views::Widget {
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
 
+  // ShelfBackgroundAnimatorObserver:
+  void UpdateShelfItemBackground(int alpha) override;
+
  private:
   void AddSystemTray();
   void AddWebNotificationTray();