Make FrameDestructionObserver::m_frame a private.

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 16 matching lines @@
 #define FrameDestructionObserver_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class LocalFrame;
 
 class FrameDestructionObserver : public WillBeGarbageCollectedMixin {
 public:
-    explicit FrameDestructionObserver(LocalFrame*);
-
     virtual void frameDestroyed();
     virtual void willDetachFrameHost();
 
     LocalFrame* frame() const { return m_frame; }
 
     virtual void trace(Visitor*);
 
 protected:
+    explicit FrameDestructionObserver(LocalFrame*);
+
 #if !ENABLE(OILPAN)
     virtual ~FrameDestructionObserver();
 #endif
 
     void observeFrame(LocalFrame*);
 
+private:
     RawPtrWillBeMember<LocalFrame> m_frame;
 };
 
-}
+} // namespace blink
 
-#endif
+#endif // FrameDestructionObserver_h