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 179 matching lines @@
     };
 
     InitParams();
     explicit InitParams(Type type);
     ~InitParams();
 
     Type type;
     // If NULL, a default implementation will be constructed.
     WidgetDelegate* delegate;
     bool child;
-    // If TRANSLUCENT_WINDOW, the widget may be fully or partially transparent.
+    // If TRANSLUCENT_WINDOW, the widget may be fully or partially transparent,
+    // or not at all if IsTranslucentWindowOpacitySupported() is false.

[sky, 2014/07/17 15:52:32]

That first sentence is meant to indicate what the consumer of widget may do
where as what you're saying applies to whether translucency is supported. Break
this up leaving the first line as one sentence with something like: 'Translucent
windows may not always be supported. Use IsTranslucentWindowOpacitySupported to
determine if translucent windows are supported.'

[jackhou1, 2014/07/18 00:41:15]

Done.

     // If OPAQUE_WINDOW, we can perform optimizations based on the widget being
     // fully opaque.  Defaults to TRANSLUCENT_WINDOW if
     // ViewsDelegate::UseTransparentWindows().  Defaults to OPAQUE_WINDOW for
     // non-window widgets.
     WindowOpacity opacity;
     bool accept_events;
     Activatable activatable;
     bool keep_on_top;
     bool visible_on_all_workspaces;
     Ownership ownership;
@@ skipping 508 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.

[sky, 2014/07/17 15:52:32]

Whether the widget supports translucency.

[jackhou1, 2014/07/18 00:41:16]

Done.

+  bool IsTranslucentWindowOpacitySupported() const;
+
   // 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_