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

Source/core/css/resolver/ElementResolveContext.cpp

 /*
  * 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 13 matching lines @@
 
 #include "core/dom/Node.h"
 #include "core/dom/NodeRenderStyle.h"
 #include "core/dom/NodeRenderingTraversal.h"
 #include "core/dom/VisitedLinkState.h"
 
 namespace WebCore {
 
 ElementResolveContext::ElementResolveContext(Element* element)
     : m_element(element)
-    , m_elementLinkState(element ? element->document()->visitedLinkState()->determineLinkState(element) : NotInsideLink)
+    , m_elementLinkState(element ? element->document().visitedLinkState()->determineLinkState(element) : NotInsideLink)
     , m_distributedToInsertionPoint(false)
     , m_resetStyleInheritance(false)
 {
     NodeRenderingTraversal::ParentDetails parentDetails;
     m_parentNode = NodeRenderingTraversal::parent(element, &parentDetails);
     m_distributedToInsertionPoint = parentDetails.insertionPoint();
     m_resetStyleInheritance = parentDetails.resetStyleInheritance();
 
-    Node* documentElement = document()->documentElement();
-    RenderStyle* documentStyle = document()->renderStyle();
+    Node* documentElement = document().documentElement();
+    RenderStyle* documentStyle = document().renderStyle();
     m_rootElementStyle = documentElement && element != documentElement ? documentElement->renderStyle() : documentStyle;
 }
 
 } // namespace WebCore