Have DOMImplementation::document() return reference.

Source/core/dom/DOMImplementation.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * 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.
@@ skipping 29 matching lines @@
 class KURL;
 class XMLDocument;
 
 class DOMImplementation : public ScriptWrappable {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<DOMImplementation> create(Document& document) { return adoptPtr(new DOMImplementation(document)); }
 
     void ref() { m_document.ref(); }
     void deref() { m_document.deref(); }
-    Document* document() { return &m_document; }
+    Document& document() const { return m_document; }
 
     // DOM methods & attributes for DOMImplementation
     static bool hasFeature(const String& feature, const String& version);
     bool hasFeatureForBindings(const String& feature, const String& version);
     PassRefPtr<DocumentType> createDocumentType(const AtomicString& qualifiedName, const String& publicId, const String& systemId, ExceptionState&);
     PassRefPtr<XMLDocument> createDocument(const AtomicString& namespaceURI, const AtomicString& qualifiedName, DocumentType*, ExceptionState&);
 
     DOMImplementation* getInterface(const String& feature);
 
     // From the DOMImplementationCSS interface
@@ skipping 12 matching lines @@
 
 private:
     explicit DOMImplementation(Document&);
 
     Document& m_document;
 };
 
 } // namespace WebCore
 
 #endif