Have DOMImplementation::document() return reference.

Source/core/css/StyleSheetList.cpp

 /**
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007 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 26 matching lines @@
 }
 
 StyleSheetList::~StyleSheetList()
 {
 }
 
 inline const Vector<RefPtr<StyleSheet> >& StyleSheetList::styleSheets()
 {
     if (!m_treeScope)
         return m_detachedStyleSheets;
-    return document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
+    return document().styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
 }
 
 void StyleSheetList::detachFromDocument()
 {
-    m_detachedStyleSheets = document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
+    m_detachedStyleSheets = document().styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
     m_treeScope = 0;
 }
 
 unsigned StyleSheetList::length()
 {
     return styleSheets().size();
 }
 
 StyleSheet* StyleSheetList::item(unsigned index)
 {
@@ skipping 20 matching lines @@
 
 CSSStyleSheet* StyleSheetList::anonymousNamedGetter(const AtomicString& name)
 {
     HTMLStyleElement* item = getNamedItem(name);
     if (!item)
         return 0;
     return item->sheet();
 }
 
 } // namespace WebCore