Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/dom/IdTargetObserver.h

Index: Source/core/dom/IdTargetObserver.h
diff --git a/Source/core/dom/IdTargetObserver.h b/Source/core/dom/IdTargetObserver.h
index 22f26bfd4c13ca3784735aecf777c9effbfffcf4..1fc4cd6f57520d79965e372712e91ede0dfd34f8 100644
--- a/Source/core/dom/IdTargetObserver.h
+++ b/Source/core/dom/IdTargetObserver.h
@@ -36,6 +36,7 @@ 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);