Have Node::highestAncestor() return a reference

Source/core/html/HTMLImageElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google 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 105 matching lines @@
 const AtomicString HTMLImageElement::imageSourceURL() const
 {
     return m_bestFitImageURL.isNull() ? fastGetAttribute(srcAttr) : m_bestFitImageURL;
 }
 
 HTMLFormElement* HTMLImageElement::formOwner() const
 {
     return m_form.get();
 }
 
-void HTMLImageElement::formRemovedFromTree(const Node* formRoot)
+void HTMLImageElement::formRemovedFromTree(const Node& formRoot)
 {
     ASSERT(m_form);
     if (highestAncestor() != formRoot)
         resetFormOwner();
 }
 
 void HTMLImageElement::resetFormOwner()
 {
     m_formWasSetByParser = false;
     HTMLFormElement* nearestForm = findFormAncestor();
@@ skipping 267 matching lines @@
 
     return m_imageLoader.image()->image();
 }
 
 bool HTMLImageElement::isInteractiveContent() const
 {
     return fastHasAttribute(usemapAttr);
 }
 
 }