Oilpan: have LifetimeNotifier<T> track its observers weakly.

Source/platform/LifecycleObserver.h

 /*
  * Copyright (C) 2008 Apple 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 16 matching lines @@
 #ifndef LifecycleObserver_h
 #define LifecycleObserver_h
 
 #include "platform/heap/Handle.h"
 #include "wtf/Assertions.h"
 
 namespace blink {
 
 template<typename T, typename Observer, typename Notifier>
 class LifecycleObserver : public WillBeGarbageCollectedMixin {
-    // FIXME: Oilpan: Remove the pre-finalizer by moving LifecycleNotifier
-    // to Oilpan's heap and making LifecycleNotifier::m_observers
-    // a hash set of weak members.
-    WILL_BE_USING_PRE_FINALIZER(LifecycleObserver, dispose);
 public:
     using Context = T;
 
     explicit LifecycleObserver(Context* context)
         : m_lifecycleContext(nullptr)
     {
-#if ENABLE(OILPAN)
-        ThreadState::current()->registerPreFinalizer(*this);
-#endif
     }
 
     virtual ~LifecycleObserver()
     {
 #if !ENABLE(OILPAN)
         dispose();
 #endif
     }
 
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
         visitor->trace(m_lifecycleContext);
     }
     virtual void contextDestroyed() { }
+
     void dispose()
     {
         setContext(nullptr);
     }
 
     Context* lifecycleContext() const { return m_lifecycleContext; }
     void clearLifecycleContext() { m_lifecycleContext = nullptr; }
 
 protected:
     void setContext(Context*);
@@ skipping 10 matching lines @@
 
     m_lifecycleContext = context;
 
     if (m_lifecycleContext)
         static_cast<Notifier*>(m_lifecycleContext.get())->addObserver(static_cast<Observer*>(this));
 }
 
 } // namespace blink
 
 #endif // LifecycleObserver_h