Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/dom/shadow/ShadowRoot.idl

 /*
  * Copyright (C) 2012 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.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 18 matching lines @@
 
     [ImplementedAs=olderShadowRootForBindings] readonly attribute ShadowRoot olderShadowRoot;
 
     [TreatNullAs=NullString, CustomElementCallbacks, RaisesException=Setter] attribute DOMString innerHTML;
 
     [RaisesException] Node cloneNode([Default=Undefined] optional boolean deep);
     Selection getSelection();
     Element getElementById([Default=Undefined] optional DOMString elementId);
     HTMLCollection getElementsByClassName([Default=Undefined] optional DOMString className);
     HTMLCollection getElementsByTagName([Default=Undefined] optional DOMString tagName);
-    HTMLCollection getElementsByTagNameNS([TreatNullAs=NullString,Default=Undefined] optional DOMString namespaceURI,
+    HTMLCollection getElementsByTagNameNS([Default=Undefined] optional DOMString? namespaceURI,
                                           [Default=Undefined] optional DOMString localName);
 
     Element elementFromPoint([Default=Undefined] optional long x,
                              [Default=Undefined] optional long y);
 
     readonly attribute StyleSheetList styleSheets;
     readonly attribute Element host;
 };