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

Source/core/html/HTMLMapElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 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 63 matching lines @@
 
     if (defaultArea) {
         result.setInnerNode(defaultArea);
         result.setURLElement(defaultArea);
     }
     return defaultArea;
 }
 
 HTMLImageElement* HTMLMapElement::imageElement()
 {
-    RefPtr<HTMLCollection> images = document()->images();
+    RefPtr<HTMLCollection> images = document().images();
     for (unsigned i = 0; Node* curr = images->item(i); i++) {
         if (!curr->hasTagName(imgTag))
             continue;
 
         // The HTMLImageElement's useMap() value includes the '#' symbol at the beginning,
         // which has to be stripped off.
         HTMLImageElement* imageElement = toHTMLImageElement(curr);
         String useMapName = imageElement->getAttribute(usemapAttr).string().substring(1);
         if (equalIgnoringCase(useMapName, m_name))
             return imageElement;
     }
 
     return 0;
 }
 
 void HTMLMapElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     // FIXME: This logic seems wrong for XML documents.
     // Either the id or name will be used depending on the order the attributes are parsed.
 
     if (isIdAttributeName(name) || name == nameAttr) {
         if (isIdAttributeName(name)) {
             // Call base class so that hasID bit gets set.
             HTMLElement::parseAttribute(name, value);
-            if (document()->isHTMLDocument())
+            if (document().isHTMLDocument())
                 return;
         }
         if (inDocument())
             treeScope()->removeImageMap(this);
         String mapName = value;
         if (mapName[0] == '#')
             mapName = mapName.substring(1);
-        m_name = document()->isHTMLDocument() ? mapName.lower() : mapName;
+        m_name = document().isHTMLDocument() ? mapName.lower() : mapName;
         if (inDocument())
             treeScope()->addImageMap(this);
 
         return;
     }
 
     HTMLElement::parseAttribute(name, value);
 }
 
 PassRefPtr<HTMLCollection> HTMLMapElement::areas()
 {
     return ensureCachedHTMLCollection(MapAreas);
 }
 
 Node::InsertionNotificationRequest HTMLMapElement::insertedInto(ContainerNode* insertionPoint)
 {
     if (insertionPoint->inDocument())
         treeScope()->addImageMap(this);
     return HTMLElement::insertedInto(insertionPoint);
 }
 
 void HTMLMapElement::removedFrom(ContainerNode* insertionPoint)
 {
     if (insertionPoint->inDocument())
         treeScope()->removeImageMap(this);
     HTMLElement::removedFrom(insertionPoint);
 }
 
 }