Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/widget.h

Index: ui/views/widget/widget.h
diff --git a/ui/views/widget/widget.h b/ui/views/widget/widget.h
index bc3f91473dd19de5175b0b939a69341424b7a329..0881e3e5214ef391c087d04da8c0fb9f116cfced 100644
--- a/ui/views/widget/widget.h
+++ b/ui/views/widget/widget.h
@@ -197,7 +197,8 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     // 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
@@ -726,6 +727,9 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
   // 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