Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/desktop_aura/desktop_window_tree_host.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_DESKTOP_AURA_DESKTOP_WINDOW_TREE_HOST_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_WINDOW_TREE_HOST_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/base/ui_base_types.h"
@@ skipping 135 matching lines @@
   virtual void OnRootViewLayout() = 0;
 
   // Called when the DesktopNativeWidgetAura's aura::Window is focused and
   // blurred.
   virtual void OnNativeWidgetFocus() = 0;
   virtual void OnNativeWidgetBlur() = 0;
 
   // Returns true if the Widget was closed but is still showing because of
   // animations.
   virtual bool IsAnimatingClosed() const = 0;
+
+  // Returns true if the Widget supports translucency.
+  virtual bool IsTranslucentWindowOpacitySupported() const = 0;
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_WINDOW_TREE_HOST_H_