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

Source/core/css/StyleSheetContents.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2012 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 369 matching lines @@
     StyleSheetContents* root = rootStyleSheet();
     if (root->m_clients.isEmpty())
         return 0;
     ASSERT(root->m_clients.size() == 1);
     return root->m_clients[0]->ownerNode();
 }
 
 Document* StyleSheetContents::singleOwnerDocument() const
 {
     Node* ownerNode = singleOwnerNode();
-    return ownerNode ? ownerNode->document() : 0;
+    return ownerNode ? &ownerNode->document() : 0;
 }
 
 KURL StyleSheetContents::completeURL(const String& url) const
 {
     return CSSParser::completeURL(m_parserContext, url);
 }
 
 void StyleSheetContents::addSubresourceStyleURLs(ListHashSet<KURL>& urls)
 {
     Deque<StyleSheetContents*> styleSheetQueue;
@@ skipping 98 matching lines @@
     m_isInMemoryCache = false;
 }
 
 void StyleSheetContents::shrinkToFit()
 {
     m_importRules.shrinkToFit();
     m_childRules.shrinkToFit();
 }
 
 }