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

Source/core/rendering/svg/RenderSVGResource.cpp

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) Research In Motion Limited 2010. 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 156 matching lines @@
 
         if (RenderSVGResourceMasker* masker = resources->masker())
             masker->removeClientFromCache(object);
 
         if (RenderSVGResourceClipper* clipper = resources->clipper())
             clipper->removeClientFromCache(object);
     }
 
     if (!object->node() || !object->node()->isSVGElement())
         return;
-    HashSet<SVGElement*>* dependencies = object->document()->accessSVGExtensions()->setOfElementsReferencingTarget(toSVGElement(object->node()));
+    HashSet<SVGElement*>* dependencies = object->document().accessSVGExtensions()->setOfElementsReferencingTarget(toSVGElement(object->node()));
     if (!dependencies)
         return;
     HashSet<SVGElement*>::iterator end = dependencies->end();
     for (HashSet<SVGElement*>::iterator it = dependencies->begin(); it != end; ++it) {
         if (RenderObject* renderer = (*it)->renderer())
             RenderSVGResource::markForLayoutAndParentResourceInvalidation(renderer, needsLayout);
     }
 }
 
 void RenderSVGResource::markForLayoutAndParentResourceInvalidation(RenderObject* object, bool needsLayout)
 {
     ASSERT(object);
-    ASSERT(object->document());
     ASSERT(object->node());
 
     if (needsLayout)
         object->setNeedsLayout();
 
     removeFromCacheAndInvalidateDependencies(object, needsLayout);
 
     // Invalidate resources in ancestor chain, if needed.
     RenderObject* current = object->parent();
     while (current) {
         removeFromCacheAndInvalidateDependencies(current, needsLayout);
 
         if (current->isSVGResourceContainer()) {
             // This will process the rest of the ancestors.
             current->toRenderSVGResourceContainer()->removeAllClientsFromCache();
             break;
         }
 
         current = current->parent();
     }
 }
 
 }