Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/ui/native_app_window.h

 // Copyright 2013 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 APPS_UI_NATIVE_APP_WINDOW_H_
 #define APPS_UI_NATIVE_APP_WINDOW_H_
 
 #include "apps/app_window.h"
 #include "components/web_modal/web_contents_modal_dialog_host.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ skipping 69 matching lines @@
   // Returns the minimum size constraints of the content.
   virtual gfx::Size GetContentMinimumSize() const = 0;
 
   // Returns the maximum size constraints of the content.
   virtual gfx::Size GetContentMaximumSize() const = 0;
 
   // Updates the minimum and maximum size constraints of the content.
   virtual void SetContentSizeConstraints(const gfx::Size& min_size,
                                          const gfx::Size& max_size) = 0;
 
+  // Returns false if the underlying native window ignores alpha transparency
+  // when compositing.
+  virtual bool CanHaveAlphaEnabled() const = 0;
+
   virtual ~NativeAppWindow() {}
 };
 
 }  // namespace apps
 
 #endif  // APPS_UI_NATIVE_APP_WINDOW_H_