Remove CanvasRenderingContext from the web exposed objects

Source/core/dom/Document.idl

 /*
  * Copyright (C) 2006, 2007, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006, 2007 Samuel Weinig <sam@webkit.org>
  *
  * 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 123 matching lines @@
 
     // Mozilla extensions
     Selection          getSelection();
     [TreatReturnedNullStringAs=Null] readonly attribute DOMString characterSet;
 
     // WebKit extensions
 
     [TreatReturnedNullStringAs=Null] readonly attribute DOMString preferredStylesheetSet;
              [TreatReturnedNullStringAs=Null, TreatNullAs=NullString] attribute DOMString selectedStylesheetSet;
 
-    CanvasRenderingContext getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);
+    (CanvasRenderingContext2D or WebGLRenderingContext) getCSSCanvasContext(DOMString contextId, DOMString name, long width, long height);
 
     // HTML 5
     HTMLCollection getElementsByClassName(DOMString classNames);
     readonly attribute Element activeElement;
     boolean hasFocus();
 
     readonly attribute DOMString compatMode;
 
     // NodeSelector - Selector API
     [RaisesException] Element querySelector(DOMString selectors);
@@ skipping 51 matching lines @@
     // document in the console. It's possible http://crbug.com/43394 will resolve this.
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=visibilityState] readonly attribute DOMString webkitVisibilityState;
     [MeasureAs=PrefixedPageVisibility, ImplementedAs=hidden] readonly attribute boolean webkitHidden;
 
     readonly attribute HTMLScriptElement currentScript;
 };
 
 Document implements GlobalEventHandlers;
 Document implements ParentNode;