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

Source/core/html/HTMLOptGroupElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * 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
@@ skipping 107 matching lines @@
 PassRefPtr<RenderStyle> HTMLOptGroupElement::customStyleForRenderer()
 {
     // styleForRenderer is called whenever a new style should be associated
     // with an Element so now is a good time to update our cached style.
     updateNonRenderStyle();
     return m_style;
 }
 
 String HTMLOptGroupElement::groupLabelText() const
 {
-    String itemText = document()->displayStringModifiedByEncoding(getAttribute(labelAttr));
+    String itemText = document().displayStringModifiedByEncoding(getAttribute(labelAttr));
 
     // In WinIE, leading and trailing whitespace is ignored in options and optgroups. We match this behavior.
     itemText = itemText.stripWhiteSpace();
     // We want to collapse our whitespace too.  This will match other browsers.
     itemText = itemText.simplifyWhiteSpace();
 
     return itemText;
 }
 
 HTMLSelectElement* HTMLOptGroupElement::ownerSelectElement() const
@@ skipping 10 matching lines @@
 
 void HTMLOptGroupElement::accessKeyAction(bool)
 {
     HTMLSelectElement* select = ownerSelectElement();
     // send to the parent to bring focus to the list box
     if (select && !select->focused())
         select->accessKeyAction(false);
 }
 
 } // namespace