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 b4dd3d997d78ad67af154485addaa9cc3b0d17dd..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; }