Add forEach() to CSSVariablesMap

Source/core/css/CSSVariablesMap.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 14 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 interface CSSVariablesMap {
     readonly attribute unsigned long size;
     DOMString get(DOMString name);
     boolean has(DOMString name);
     [RaisesException] void set(DOMString name, DOMString value);
     [ImplementedAs=remove] boolean delete(DOMString name);
     [RaisesException] void clear();
+    void forEach(CSSVariablesMapForEachCallback callback, optional any thisArg);
 };