[Binding] Support primitive type for union member

Source/core/dom/DOMStringMap.idl

 /*
  * Copyright (C) 2010 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 11 matching lines @@
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
  * THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
     GenerateIsReachable=element,
     CustomDeleteProperty,
     CustomEnumerateProperty,
     CustomNamedSetter,
 ] interface DOMStringMap {
-    [ImplementedAs=item, DoNotCheckJSProperty] getter DOMString (DOMString name);
+    [ImplementedAs=item, OverrideBuiltins] getter DOMString (DOMString name);
 };