Oilpan: remove unused support for removing FrameDestructionObservers.

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 13e77dfd046e178cc1b8abbd1d4bc3845f8b5443..9c6175e4d61f5be6178c5f434449c2574efb2d11 100644
--- a/Source/core/frame/FrameDestructionObserver.h
+++ b/Source/core/frame/FrameDestructionObserver.h
@@ -55,9 +55,11 @@ protected:
 #if !ENABLE(OILPAN)
     virtual ~FrameDestructionObserver();
 #endif
-    void observeFrame(LocalFrame*);
 
     RawPtrWillBeWeakMember<LocalFrame> m_frame;
+
+private:
+    void observeFrame(LocalFrame*);
 };
 
 }