Add app.window.alphaEnabled() and onAlphaEnabledChanged.

chrome/common/extensions/api/app_current_window_internal.idl

 // 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.
 
 // This is used by the app window API internally to pass through messages to
 // the shell window.
 [nodoc] namespace app.currentWindowInternal {
 
   // Null or undefined indicates that a value should not change.
   dictionary Bounds {
@@ skipping 43 matching lines @@
     static void setAlwaysOnTop(boolean always_on_top);
   };
 
   interface Events {
     static void onClosed();
     static void onBoundsChanged();
     static void onFullscreened();
     static void onMinimized();
     static void onMaximized();
     static void onRestored();
+    static void onAlphaEnabledChanged();
     // Only sent in tests.
     static void onWindowShownForTests();
   };
 };