Oilpan: Move LifecycleNotifier's hierarchy to Oilpan's heap

Source/platform/LifecycleObserver.h

Index: Source/platform/LifecycleObserver.h
diff --git a/Source/platform/LifecycleObserver.h b/Source/platform/LifecycleObserver.h
index 44b5d2a345379cb149506772b252de4000401b4b..d4cd3f3b1827e202e552c1498c3a88398309588a 100644
--- a/Source/platform/LifecycleObserver.h
+++ b/Source/platform/LifecycleObserver.h
@@ -34,10 +34,6 @@ namespace blink {
 
 template<typename T>
 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:
     typedef T Context;
 
@@ -53,15 +49,12 @@ public:
         : m_lifecycleContext(nullptr)
         , m_observerType(type)
     {
-#if ENABLE(OILPAN)
-        ThreadState::current()->registerPreFinalizer(*this);
-#endif
         setContext(context);
     }
     virtual ~LifecycleObserver()
     {
 #if !ENABLE(OILPAN)
-        dispose();
+        setContext(nullptr);
 #endif
     }
 
@@ -70,10 +63,6 @@ public:
         visitor->trace(m_lifecycleContext);
     }
     virtual void contextDestroyed() { }
-    void dispose()
-    {
-        setContext(nullptr);
-    }
 
     Context* lifecycleContext() const { return m_lifecycleContext; }
     void clearLifecycleContext() { m_lifecycleContext = nullptr; }