Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/dom/IdTargetObserver.h

 /*
  * Copyright (C) 2012 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:
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 18 matching lines @@
 #include "wtf/text/AtomicString.h"
 
 namespace WebCore {
 
 class IdTargetObserverRegistry;
 
 class IdTargetObserver {
 public:
     virtual ~IdTargetObserver();
     virtual void idTargetChanged() = 0;
+    virtual void unregister();
 
 protected:
     IdTargetObserver(IdTargetObserverRegistry&, const AtomicString& id);
 
 private:
     IdTargetObserverRegistry& m_registry;
     AtomicString m_id;
 };
 
 } // namespace WebCore
 
 #endif // IdTargetObserver_h