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

Source/core/dom/StyleSheetCollection.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 75 matching lines @@
 
     DocumentOrderedList& styleSheetCandidateNodes() { return m_styleSheetCandidateNodes; }
     DocumentOrderedList* scopingNodesForStyleScoped() { return m_scopingNodesForStyleScoped.scopingNodes(); }
     ListHashSet<Node*, 4>* scopingNodesRemoved() { return m_scopingNodesForStyleScoped.scopingNodesRemoved(); }
 
     void clearMediaQueryRuleSetStyleSheets();
 
 protected:
     explicit StyleSheetCollection(TreeScope&);
 
-    Document* document() { return m_treeScope.documentScope(); }
+    // FIXME: Should return a reference.
+    Document* document() { return &m_treeScope.documentScope(); }
 
     enum StyleResolverUpdateType {
         Reconstruct,
         Reset,
         Additive,
         ResetStyleResolverAndFontSelector
     };
 
     struct StyleSheetChange {
         StyleResolverUpdateType styleResolverUpdateType;
@@ skipping 18 matching lines @@
     bool m_usesRemUnits;
 
     DocumentOrderedList m_styleSheetCandidateNodes;
     StyleSheetScopingNodeList m_scopingNodesForStyleScoped;
 };
 
 }
 
 #endif