Move app.currentWindowInternal API implementation to extensions

extensions/common/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 49 matching lines @@
     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();
   };
 };