IDL: Support optional argument default value syntax

Source/bindings/tests/idls/TestInterfaceConstructor2.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 26 matching lines @@
 // TestInterfaceConstructor.idl), otherwise overload resolution check string is
 // extremely long and triggers a lint warning (line length).
 [
     // 2 constructors with same type length, to test overload resolution
     Constructor(DOMString stringArg),
     Constructor(Dictionary dictionaryArg),
     Constructor(
         TestInterfaceEmpty testInterfaceEmptyArg,
         long longArg,
         [Default=Undefined] optional DOMString defaultUndefinedOptionalStringArg,
-        [Default=NullString] optional DOMString defaultNullStringOptionalStringArg,
+        optional DOMString defaultNullStringOptionalStringArg = null,
         [Default=Undefined] optional Dictionary defaultUndefinedOptionalDictionaryArg,
         optional DOMString optionalStringArg),
 ] interface TestInterfaceConstructor2 {
 };