Oilpan: Replace RefPtr to Node in MutationObserver-related classes with Oilpan transition types.

Source/core/dom/MutationObserverRegistration.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 128 matching lines @@
         return;
     for (NodeHashSet::const_iterator iter = m_transientRegistrationNodes->begin(); iter != m_transientRegistrationNodes->end(); ++iter)
         nodes.add(iter->get());
 }
 
 void MutationObserverRegistration::trace(Visitor* visitor)
 {
     visitor->trace(m_observer);
     visitor->trace(m_registrationNode);
     visitor->trace(m_registrationNodeKeepAlive);
+    visitor->trace(m_transientRegistrationNodes);
 }
 
 } // namespace WebCore