Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/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 UI_VIEWS_WIDGET_WIDGET_H_
 #define UI_VIEWS_WIDGET_WIDGET_H_
 
 #include <set>
 #include <stack>
 #include <vector>
@@ skipping 708 matching lines @@
   gfx::Rect GetWorkAreaBoundsInScreen() const;
 
   // Creates and dispatches synthesized mouse move event using the current
   // mouse location to refresh hovering status in the widget.
   void SynthesizeMouseMoveEvent();
 
   // Called by our RootView after it has performed a Layout. Used to forward
   // window sizing information to the window server on some platforms.
   void OnRootViewLayout();
 
+  // Whether the widget can have a transparent background.
+  bool CanHaveAlphaEnabled() const;

[sky, 2014/07/16 15:14:54]

This name is confusing. No where in widget do we have anything named 'alpha'. I
think you want IsTranslucentWindowOpacity supported, which better matches what
you're tracking. You'll want to update the comments above WindowOpacity to see
this method.

[jackhou1, 2014/07/17 03:07:23]

Done.

+
   // Notification that our owner is closing.
   // NOTE: this is not invoked for aura as it's currently not needed there.
   // Under aura menus close by way of activation getting reset when the owner
   // closes.
   virtual void OnOwnerClosing();
 
   // Overridden from NativeWidgetDelegate:
   virtual bool IsModal() const OVERRIDE;
   virtual bool IsDialogBox() const OVERRIDE;
   virtual bool CanActivate() const OVERRIDE;
@@ skipping 189 matching lines @@
   bool movement_disabled_;
 
   ScopedObserver<ui::NativeTheme, ui::NativeThemeObserver> observer_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_H_