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

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 621 matching lines @@
     bool hasPendingForcedStyleRecalc() const;
     void styleRecalcTimerFired(Timer<Document>*);
 
     void registerNodeList(LiveNodeListBase*);
     void unregisterNodeList(LiveNodeListBase*);
     bool shouldInvalidateNodeListCaches(const QualifiedName* attrName = 0) const;
     void invalidateNodeListCaches(const QualifiedName* attrName);
 
     void attachNodeIterator(NodeIterator*);
     void detachNodeIterator(NodeIterator*);
-    void moveNodeIteratorsToNewDocument(Node*, Document*);
+    void moveNodeIteratorsToNewDocument(Node&, Document&);
 
     void attachRange(Range*);
     void detachRange(Range*);
 
     void updateRangesAfterChildrenChanged(ContainerNode*);
     // nodeChildrenWillBeRemoved is used when removing all node children at once.
     void nodeChildrenWillBeRemoved(ContainerNode*);
     // nodeWillBeRemoved is only safe when removing one node at a time.
     void nodeWillBeRemoved(Node&);
     bool canReplaceChild(const Node& newChild, const Node& oldChild) const;
@@ skipping 697 matching lines @@
 inline bool Node::isDocumentNode() const
 {
     return this == document();
 }
 
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace WebCore
 
 #endif // Document_h