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

Source/core/html/HTMLAnchorElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  * Copyright (C) 2007, 2008, 2009, 2010 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.
@@ skipping 131 matching lines @@
     bool m_hasRootEditableElementForSelectionOnMouseDown : 1;
     bool m_wasShiftKeyDownOnMouseDown : 1;
     uint32_t m_linkRelations : 30;
     OwnPtr<PrefetchEventHandler> m_prefetchEventHandler;
     mutable LinkHash m_cachedVisitedLinkHash;
 };
 
 inline LinkHash HTMLAnchorElement::visitedLinkHash() const
 {
     if (!m_cachedVisitedLinkHash)
-        m_cachedVisitedLinkHash = WebCore::visitedLinkHash(document()->baseURL(), fastGetAttribute(HTMLNames::hrefAttr));
+        m_cachedVisitedLinkHash = WebCore::visitedLinkHash(document().baseURL(), fastGetAttribute(HTMLNames::hrefAttr));
     return m_cachedVisitedLinkHash;
 }
 
 // Functions shared with the other anchor elements (i.e., SVG).
 
 bool isEnterKeyKeydownEvent(Event*);
 bool isLinkClick(Event*);
 
 inline bool isHTMLAnchorElement(const Node* node)
 {
     return node->hasTagName(HTMLNames::aTag);
 }
 
 inline bool isHTMLAnchorElement(const Element* element)
 {
     return element->hasTagName(HTMLNames::aTag);
 }
 
 inline HTMLAnchorElement* toHTMLAnchorElement(Node* node)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!node || isHTMLAnchorElement(node));
     return static_cast<HTMLAnchorElement*>(node);
 }
 
 } // namespace WebCore
 
 #endif // HTMLAnchorElement_h