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

ash/common/system/status_area_widget.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_COMMON_SYSTEM_STATUS_AREA_WIDGET_H_
 #define ASH_COMMON_SYSTEM_STATUS_AREA_WIDGET_H_
 
 #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"
 
 namespace ash {
 class OverviewButtonTray;
 class ShellDelegate;
 class StatusAreaWidgetDelegate;
 class SystemTray;
 class WebNotificationTray;
 class WmShelf;
 class WmWindow;
 #if defined(OS_CHROMEOS)
 class ImeMenuTray;
 class LogoutButtonTray;
 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;
 
   // Creates the SystemTray, WebNotificationTray and LogoutButtonTray.
   void CreateTrayViews();
 
   // Destroys the system tray and web notification tray. Called before
   // tearing down the windows to avoid shutdown ordering issues.
   void Shutdown();
@@ skipping 33 matching lines @@
 
   // Notifies child trays, and the |status_area_widget_delegate_| to schedule a
   // paint.
   void SchedulePaint();
 
   // Overridden from views::Widget:
   void OnNativeWidgetActivationChanged(bool active) override;
   void OnMouseEvent(ui::MouseEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
 
+  // ShelfBackgroundAnimatorObserver:
+  void UpdateShelfItemBackground(int alpha) override;
+
  private:
   void AddSystemTray();
   void AddWebNotificationTray();
 #if defined(OS_CHROMEOS)
   void AddLogoutButtonTray();
   void AddPaletteTray();
   void AddVirtualKeyboardTray();
   void AddImeMenuTray();
 #endif
   void AddOverviewButtonTray();
@@ skipping 12 matching lines @@
   LoginStatus login_status_;
 
   WmShelf* wm_shelf_;
 
   DISALLOW_COPY_AND_ASSIGN(StatusAreaWidget);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_STATUS_AREA_WIDGET_H_