Have Node::document() return a reference instead of a pointer

Source/core/dom/DocumentType.cpp

 /*
  * 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, 2009 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 36 matching lines @@
     return name();
 }
 
 Node::NodeType DocumentType::nodeType() const
 {
     return DOCUMENT_TYPE_NODE;
 }
 
 PassRefPtr<Node> DocumentType::cloneNode(bool /*deep*/)
 {
-    return create(document(), m_name, m_publicId, m_systemId);
+    return create(&document(), m_name, m_publicId, m_systemId);
 }
 
 Node::InsertionNotificationRequest DocumentType::insertedInto(ContainerNode* insertionPoint)
 {
     Node::insertedInto(insertionPoint);
 
     // DocumentType can only be inserted into a Document.
     ASSERT(parentNode()->isDocumentNode());
 
-    document()->setDoctype(this);
+    document().setDoctype(this);
 
     return InsertionDone;
 }
 
 void DocumentType::removedFrom(ContainerNode* insertionPoint)
 {
-    document()->setDoctype(0);
+    document().setDoctype(0);
     Node::removedFrom(insertionPoint);
 }
 
 }