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();

[haraken, 2014/05/06 15:59:42]

Thanks to your comments, I understand how unregister() works but it's
complicated. Shall we add a FIXME and mention that IdTargetObserver should be
moved to the heap and remove the explicit unregistration?

[Mads Ager (chromium), 2014/05/07 12:13:16]

It is unclear to me exactly what the semantics should be. Currently, the
IdTargetObservers are in OwnPtrs, so the unregistration happens without delay
when the OwnPtr is overwritten. That behavior is maintained with the explicit
unregistration. We could make IdTargetObservers heap allocated and use weak
processing, but that would leave observers in the set until the next GC which is
a change in behavior from what is happening now.

 
 protected:
     IdTargetObserver(IdTargetObserverRegistry&, const AtomicString& id);
 
 private:
     IdTargetObserverRegistry& m_registry;
     AtomicString m_id;
 };
 
 } // namespace WebCore
 
 #endif // IdTargetObserver_h