Add support for BeforeUnloadEvent

Source/core/dom/Event.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 51 matching lines @@
      void initEvent([Default=Undefined] optional DOMString eventTypeArg,
                                   [Default=Undefined] optional boolean canBubbleArg,
                                   [Default=Undefined] optional boolean cancelableArg);
 
     // DOM Level 3 Additions.
     readonly attribute boolean defaultPrevented;
     void stopImmediatePropagation();
 
     // IE Extensions
     readonly attribute EventTarget      srcElement;
-             attribute boolean          returnValue;
+    [ImplementedAs=legacyReturnValue, DeprecateAs=EventReturnValue] attribute boolean returnValue;
              attribute boolean          cancelBubble;
 
     [EnabledAtRuntime=ShadowDOM] readonly attribute NodeList path;
 
     [Custom] readonly attribute Clipboard        clipboardData;
 };