Add support for DOM4's XMLDocument interface

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,
@@ skipping 14 matching lines @@
     // DOM Level 1
 
     [ImplementedAs=hasFeatureForBindings, MeasureAs=DOMImplementationHasFeature]
     boolean hasFeature(DOMString feature, [TreatNullAs=NullString] DOMString version);
 
     // DOM Level 2
 
     [RaisesException] DocumentType createDocumentType(DOMString qualifiedName,
                                                       DOMString publicId,
                                                       DOMString systemId);
-    [RaisesException] Document createDocument([TreatNullAs=NullString] DOMString namespaceURI,
-                                              [TreatNullAs=NullString] DOMString qualifiedName,
-                                              [Default=Undefined] optional DocumentType doctype);
+    [RaisesException] XMLDocument createDocument([TreatNullAs=NullString] DOMString namespaceURI,
+                                                 [TreatNullAs=NullString] DOMString qualifiedName,
+                                                 [Default=Undefined] optional DocumentType doctype);
 
     // DOMImplementationCSS interface from DOM Level 2 CSS
 
     [MeasureAs=DOMImplementationCreateCSSStyleSheet]
     CSSStyleSheet createCSSStyleSheet([Default=Undefined] optional DOMString title,
                                       [Default=Undefined] optional DOMString media);
 
     // HTMLDOMImplementation interface from DOM Level 2 HTML
 
     HTMLDocument createHTMLDocument([Default=NullString] optional DOMString title);
 };