Revert of Issue FrameDestructionObserver::frameDestroyed() notification on detach.

Source/core/frame/FrameDestructionObserver.h

 /*
  * Copyright (C) 2011 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 "platform/heap/Handle.h"
 
 namespace blink {
 
 class LocalFrame;
 
 class FrameDestructionObserver : public WillBeGarbageCollectedMixin {
 public:
     explicit FrameDestructionObserver(LocalFrame*);
 
+#if !ENABLE(OILPAN)
+    // Oilpan: there is no known need for directly observing
+    // frameDestroyed() with Oilpan enabled, as its clearing is
+    // handled by keeping a weak reference.
+    //
+    // A weak callback version of frameDestroyed() can be reintroduced
+    // later, should the need arise.
     virtual void frameDestroyed();
+#endif
     virtual void willDetachFrameHost();
 
     LocalFrame* frame() const { return m_frame; }
 
     virtual void trace(Visitor*);
 
 protected:
 #if !ENABLE(OILPAN)
     virtual ~FrameDestructionObserver();
 #endif
 
     void observeFrame(LocalFrame*);
 
-    RawPtrWillBeMember<LocalFrame> m_frame;
+    RawPtrWillBeWeakMember<LocalFrame> m_frame;
 };
 
 }
 
 #endif