Make FrameDestructionObserver::m_frame a private.

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..175f273647b639671255e583f13190f86723d09d 100644
--- a/Source/core/frame/FrameDestructionObserver.h
+++ b/Source/core/frame/FrameDestructionObserver.h
@@ -34,8 +34,6 @@ class LocalFrame;
 
 class FrameDestructionObserver : public WillBeGarbageCollectedMixin {
 public:
-    explicit FrameDestructionObserver(LocalFrame*);
-
     virtual void frameDestroyed();
     virtual void willDetachFrameHost();
 
@@ -44,15 +42,18 @@ public:
     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