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

Source/core/css/ElementRuleCollector.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 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 87 matching lines @@
     void collectMatchingRulesForRegion(const MatchRequest&, RuleRange&, CascadeScope = ignoreCascadeScope, CascadeOrder = ignoreCascadeOrder);
     void sortAndTransferMatchedRules();
     void clearMatchedRules();
     void addElementStyleProperties(const StylePropertySet*, bool isCacheable = true);
 
     unsigned lastMatchedRulesPosition() const { return m_matchedRules ? m_matchedRules->size() : 0; }
     void sortMatchedRulesFrom(unsigned position);
     void sortAndTransferMatchedRulesWithOnlySortBySpecificity();
 
 private:
-    Document* document() { return m_context.document(); }
+    Document& document() { return m_context.document(); }
 
     void collectRuleIfMatches(const RuleData&, CascadeScope, CascadeOrder, const MatchRequest&, RuleRange&);
     void collectMatchingRulesForList(const Vector<RuleData>*, CascadeScope, CascadeOrder, const MatchRequest&, RuleRange&);
     void collectMatchingRulesForList(const RuleData*, CascadeScope, CascadeOrder, const MatchRequest&, RuleRange&);
     bool ruleMatches(const RuleData&, const ContainerNode* scope, PseudoId&);
 
     void sortMatchedRules();
     void addMatchedRule(const RuleData*, CascadeScope, CascadeOrder);
 
     StaticCSSRuleList* ensureRuleList();
@@ skipping 14 matching lines @@
     OwnPtr<Vector<MatchedRule, 32> > m_matchedRules;
 
     // Output.
     RefPtr<StaticCSSRuleList> m_ruleList;
     MatchResult m_result;
 };
 
 } // namespace WebCore
 
 #endif // ElementRuleCollector_h