IDL: Support 'stringifier' keyword

Source/bindings/tests/idls/TestInterface2.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 31 matching lines @@
 ] interface TestInterface2 {
     // Indexed property operations with an identifier
     [RaisesException] getter TestInterfaceEmpty item(unsigned long index);
     [RaisesException] setter DOMString setItem(unsigned long index, DOMString value);
     [RaisesException] deleter boolean deleteItem(unsigned long index);
 
     // Named property operations with an identifier
     [RaisesException] getter TestInterfaceEmpty namedItem(DOMString name);
     [RaisesException] setter DOMString setNamedItem(DOMString name, DOMString value);
     [RaisesException] deleter boolean deleteNamedItem(DOMString name);
+
+    stringifier DOMString stringifierMethod();
 };