IDL: Support 'stringifier' keyword

Source/bindings/tests/idls/TestInterface.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 51 matching lines @@
 
     // Anonymous indexed property operations
     getter DOMString (unsigned long index);
     setter DOMString (unsigned long index, DOMString value);
     deleter boolean (unsigned long index);
 
     // Anonymous named property operations
     getter DOMString (DOMString name);
     setter DOMString (DOMString name, DOMString value);
     deleter boolean (DOMString name);
+
+    [NotEnumerable] stringifier;
 };
 
 TestInterface implements TestImplements;
 // TestInterface implements TestImplements2; // at implement*ed* interface
 TestInterface implements TestImplements3;