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

Source/core/rendering/RenderObjectChildList.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
@@ skipping 91 matching lines @@
 
     oldChild->setPreviousSibling(0);
     oldChild->setNextSibling(0);
     oldChild->setParent(0);
 
     // rendererRemovedFromTree walks the whole subtree. We can improve performance
     // by skipping this step when destroying the entire tree.
     if (!owner->documentBeingDestroyed())
         RenderCounter::rendererRemovedFromTree(oldChild);
 
-    if (AXObjectCache* cache = owner->document()->existingAXObjectCache())
+    if (AXObjectCache* cache = owner->document().existingAXObjectCache())
         cache->childrenChanged(owner);
 
     return oldChild;
 }
 
 void RenderObjectChildList::insertChildNode(RenderObject* owner, RenderObject* newChild, RenderObject* beforeChild, bool notifyRenderer)
 {
     ASSERT(!newChild->parent());
     ASSERT(!owner->isBlockFlow() || (!newChild->isTableSection() && !newChild->isTableRow() && !newChild->isTableCell()));
 
@@ skipping 31 matching lines @@
         newChild->insertedIntoTree();
 
     if (!owner->documentBeingDestroyed()) {
         RenderCounter::rendererSubtreeAttached(newChild);
     }
 
     newChild->setNeedsLayoutAndPrefWidthsRecalc();
     if (!owner->normalChildNeedsLayout())
         owner->setChildNeedsLayout(); // We may supply the static position for an absolute positioned child.
 
-    if (AXObjectCache* cache = owner->document()->axObjectCache())
+    if (AXObjectCache* cache = owner->document().axObjectCache())
         cache->childrenChanged(owner);
 }
 
 } // namespace WebCore