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

Source/core/rendering/RenderLayerBacking.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1201 matching lines @@
         return false;
 
     if (paintsBoxDecorations() || paintsChildren())
         return false;
 
     if (renderObject->isRenderRegion())
         return false;
 
     if (renderObject->node() && renderObject->node()->isDocumentNode()) {
         // Look to see if the root object has a non-simple background
-        RenderObject* rootObject = renderObject->document()->documentElement() ? renderObject->document()->documentElement()->renderer() : 0;
+        RenderObject* rootObject = renderObject->document().documentElement() ? renderObject->document().documentElement()->renderer() : 0;
         if (!rootObject)
             return false;
 
         RenderStyle* style = rootObject->style();
 
         // Reject anything that has a border, a border-radius or outline,
         // or is not a simple background (no background, or solid color).
         if (hasBoxDecorationsOrBackgroundImage(style))
             return false;
 
         // Now look at the body's renderer.
-        HTMLElement* body = renderObject->document()->body();
+        HTMLElement* body = renderObject->document().body();
         RenderObject* bodyObject = (body && body->hasLocalName(bodyTag)) ? body->renderer() : 0;
         if (!bodyObject)
             return false;
 
         style = bodyObject->style();
 
         if (hasBoxDecorationsOrBackgroundImage(style))
             return false;
     }
 
@@ skipping 684 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore