Make [TreatNullAs=NullString] DOMString arguments nullable instead

Source/core/dom/DOMImplementation.idl

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 [
     SetWrapperReferenceFrom=document,
     WillBeGarbageCollected,
 ] interface DOMImplementation {
 
     // DOM Level 1
 
     [ImplementedAs=hasFeatureForBindings, MeasureAs=DOMImplementationHasFeature]
-    boolean hasFeature(DOMString feature, [TreatNullAs=NullString] DOMString version);
+    boolean hasFeature(DOMString feature, DOMString? version);
 
     // DOM Level 2
 
     [RaisesException] DocumentType createDocumentType(DOMString qualifiedName,
                                                       DOMString publicId,
                                                       DOMString systemId);
-    [RaisesException] XMLDocument createDocument([TreatNullAs=NullString] DOMString namespaceURI,
-                                                 [TreatNullAs=NullString] DOMString qualifiedName,
+    [RaisesException] XMLDocument createDocument(DOMString? namespaceURI,
+                                                 DOMString? qualifiedName,
                                                  [Default=Undefined] optional DocumentType doctype);
 
     // HTMLDOMImplementation interface from DOM Level 2 HTML
 
     HTMLDocument createHTMLDocument(optional DOMString title = null);
 };