Add contextmenu attribute for HTMLElement.

Source/core/html/HTMLElement.idl

 /*
  * Copyright (C) 2006, 2007, 2009 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 22 matching lines @@
     [Reflect] attribute DOMString accessKey;
 
     // Extensions
     [TreatNullAs=NullString, CustomElementCallbacks, RaisesException=Setter, MeasureAs=HTMLElementInnerText] attribute DOMString innerText;
     [TreatNullAs=NullString, CustomElementCallbacks, RaisesException=Setter, MeasureAs=HTMLElementOuterText] attribute DOMString outerText;
 
     [RuntimeEnabled=IMEAPI] readonly attribute InputMethodContext inputMethodContext;
 
     [CustomElementCallbacks, RaisesException=Setter] attribute DOMString contentEditable;
     readonly attribute boolean isContentEditable;
+    [RuntimeEnabled=ContextMenu] attribute HTMLMenuElement? contextMenu;
 
              attribute boolean spellcheck;
 
     void click();
 };
 
 HTMLElement implements GlobalEventHandlers;