IDL: Support optional argument default value syntax

Source/modules/indexeddb/IDBIndex.idl

 /*
  * Copyright (C) 2010 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 14 matching lines @@
 
 [
     GarbageCollected
 ] interface IDBIndex {
     readonly attribute DOMString name;
     readonly attribute IDBObjectStore objectStore;
     [CallWith=ScriptState] readonly attribute any keyPath;
     readonly attribute boolean unique;
     readonly attribute boolean multiEntry;
 
-    [CallWith=ScriptState, RaisesException] IDBRequest openCursor([Default=Undefined] optional any range, [Default=NullString] optional DOMString direction);
-    [CallWith=ScriptState, RaisesException] IDBRequest openKeyCursor([Default=Undefined] optional any range, [Default=NullString] optional DOMString direction);
+    [CallWith=ScriptState, RaisesException] IDBRequest openCursor([Default=Undefined] optional any range, optional DOMString direction = null);

[jsbell, 2014/06/03 18:33:16]

Per spec, default here (and other direction params) is "next" - does everything
pass if you use that here?

+    [CallWith=ScriptState, RaisesException] IDBRequest openKeyCursor([Default=Undefined] optional any range, optional DOMString direction = null);
 
     [CallWith=ScriptState, RaisesException] IDBRequest get(any key);
     [CallWith=ScriptState, RaisesException] IDBRequest getKey(any key);
     [CallWith=ScriptState, RaisesException] IDBRequest count([Default=Undefined] optional any key);
 };