Tweak TextTrackCue/VTTCue virtual functions

Source/core/html/track/TextTrackCue.h

Index: Source/core/html/track/TextTrackCue.h
diff --git a/Source/core/html/track/TextTrackCue.h b/Source/core/html/track/TextTrackCue.h
index 706cbf87902bdc44a1376827996c78747aa2dcc3..92ac020549c298d12f8f5bae39e54ff54c27c0a5 100644
--- a/Source/core/html/track/TextTrackCue.h
+++ b/Source/core/html/track/TextTrackCue.h
@@ -81,17 +81,19 @@ public:
 
     // 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);