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

Source/core/dom/ElementRareData.h

 /*
  * Copyright (C) 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 David Smith <catfish.man@gmail.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
  * 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 254 matching lines @@
 {
     if (!element)
         return;
 
     if (element->attached())
         element->detach();
 
     ASSERT(!element->nextSibling());
     ASSERT(!element->previousSibling());
 
-    element->document()->removeFromTopLayer(element);
+    element->document().removeFromTopLayer(element);
     element->setParentOrShadowHostNode(0);
 }
 
 inline void ElementRareData::resetStyleState()
 {
     setComputedStyle(0);
     setStyleAffectedByEmpty(false);
     setChildIndex(0);
 }
 
 inline void ElementRareData::resetDynamicRestyleObservations()
 {
     setChildrenAffectedByHover(false);
     setChildrenAffectedByActive(false);
     setChildrenAffectedByDrag(false);
     setChildrenAffectedByFirstChildRules(false);
     setChildrenAffectedByLastChildRules(false);
     setChildrenAffectedByDirectAdjacentRules(false);
     setChildrenAffectedByForwardPositionalRules(false);
     setChildrenAffectedByBackwardPositionalRules(false);
 }
 
 } // namespace
 
 #endif // ElementRareData_h