Make TreeScope::documentScope() return a reference and ensure m_documentScope is never NULL

Source/core/dom/Node.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, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.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
@@ skipping 439 matching lines @@
 
     unsigned nodeIndex() const;
 
     // Returns the DOM ownerDocument attribute. This method never returns NULL, except in the case
     // of a Document node.
     Document* ownerDocument() const;
 
     // Returns the document associated with this node. A Document node returns itself.
     Document& document() const
     {
-        ASSERT(treeScope().documentScope());
-        return *treeScope().documentScope();
+        return treeScope().documentScope();
     }
 
     TreeScope& treeScope() const
     {
         ASSERT(m_treeScope);
         return *m_treeScope;
     }
 
     bool inActiveDocument() const;
 
@@ skipping 448 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showTree(const WebCore::Node*);
 void showNodePath(const WebCore::Node*);
 #endif
 
 #endif