Oilpan: Replace RefPtrs to Node and its subclasses in core/xml/ with Oilpan transition types.

Source/core/xml/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 1403 matching lines @@
 
 ExecutionContext* XMLHttpRequest::executionContext() const
 {
     return ActiveDOMObject::executionContext();
 }
 
 void XMLHttpRequest::trace(Visitor* visitor)
 {
     visitor->trace(m_responseBlob);
     visitor->trace(m_responseStream);
+    visitor->trace(m_responseDocument);
     visitor->trace(m_progressEventThrottle);
 }
 
 } // namespace WebCore