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

Source/core/frame/FrameDestructionObserver.h

Index: Source/core/frame/FrameDestructionObserver.h
diff --git a/Source/core/frame/FrameDestructionObserver.h b/Source/core/frame/FrameDestructionObserver.h
index 708dd4ef8b75d955b1782b8d443f658e1d2618de..a62d20210447fe2a5986198e606afdd637e4bbd7 100644
--- a/Source/core/frame/FrameDestructionObserver.h
+++ b/Source/core/frame/FrameDestructionObserver.h
@@ -36,7 +36,15 @@
 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; }
@@ -50,7 +58,7 @@
 
     void observeFrame(LocalFrame*);
 
-    RawPtrWillBeMember<LocalFrame> m_frame;
+    RawPtrWillBeWeakMember<LocalFrame> m_frame;
 };
 
 }