Make FrameDestructionObserver::m_frame a private.

Source/core/frame/FrameDestructionObserver.cpp

 /*
  * 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 51 matching lines @@
 void FrameDestructionObserver::willDetachFrameHost()
 {
     // Subclasses should override this function to handle this notification.
 }
 
 void FrameDestructionObserver::trace(Visitor* visitor)
 {
     visitor->trace(m_frame);
 }
 
-}
+} // namespace blink