Tweak TextTrackCue/VTTCue virtual functions

Source/core/html/track/TextTrackCue.h

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2012, 2013 Apple 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 63 matching lines @@
     using EventTarget::dispatchEvent;
     virtual bool dispatchEvent(PassRefPtr<Event>) OVERRIDE;
 
     bool isActive();
     void setIsActive(bool);
 
     virtual void updateDisplay(const IntSize& videoSize, HTMLDivElement& container) = 0;
 
     // FIXME: Consider refactoring to eliminate or merge the following three members.
     // https://code.google.com/p/chromium/issues/detail?id=322434
-    virtual void updateDisplayTree(double movieTime) { }
-    virtual void removeDisplayTree() { }
-    virtual void notifyRegionWhenRemovingDisplayTree(bool notifyRegion) { }
+    virtual void updateDisplayTree(double movieTime) = 0;
+    virtual void removeDisplayTree() = 0;
+    virtual void notifyRegionWhenRemovingDisplayTree(bool notifyRegion) = 0;
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+#ifndef NDEBUG
+    virtual String toString() const = 0;
+#endif
+
     DEFINE_ATTRIBUTE_EVENT_LISTENER(enter);
     DEFINE_ATTRIBUTE_EVENT_LISTENER(exit);
 
-    virtual String toString() const;
-
 protected:
     TextTrackCue(double start, double end);
 
     void cueWillChange();
     virtual void cueDidChange();
 
 private:
     String m_id;
     double m_startTime;
     double m_endTime;
     int m_cueIndex;
 
     TextTrack* m_track;
 
     bool m_isActive;
     bool m_pauseOnExit;
 };
 
 } // namespace WebCore
 
 #endif