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

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 414a7f5e1046a159a08a8f33dfd1d61e3066597b..562949612f0d98903f2ac0f73c4e0a6495a4c91b 100644
--- a/ash/common/system/status_area_widget.h
+++ b/ash/common/system/status_area_widget.h
@@ -7,6 +7,7 @@
 
 #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"
@@ -26,7 +27,8 @@ class PaletteTray;
 class VirtualKeyboardTray;
 #endif
 
-class ASH_EXPORT StatusAreaWidget : public views::Widget {
+class ASH_EXPORT StatusAreaWidget : public views::Widget,
+                                    public ShelfBackgroundAnimatorObserver {
  public:
   StatusAreaWidget(WmWindow* status_container, WmShelf* wm_shelf);
   ~StatusAreaWidget() override;
@@ -80,6 +82,9 @@ 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();