Implement DataCue interface.

Source/core/html/track/vtt/VTTCue.h

Index: Source/core/html/track/vtt/VTTCue.h
diff --git a/Source/core/html/track/vtt/VTTCue.h b/Source/core/html/track/vtt/VTTCue.h
index e30c4fe3acad897c3019c24e43a7658e6333e32c..4c9348be8d44a875a33c8f9c2fa8758515fcf269 100644
--- a/Source/core/html/track/vtt/VTTCue.h
+++ b/Source/core/html/track/vtt/VTTCue.h
@@ -134,6 +134,8 @@ public:
     };
     CueAlignment getAlignment() const { return m_cueAlignment; }
 
+    virtual CueType cueType() const OVERRIDE { return WebVTT; }
+
     virtual ExecutionContext* executionContext() const OVERRIDE;
 
 #ifndef NDEBUG
@@ -192,8 +194,7 @@ private:
     bool m_notifyRegion : 1;
 };
 
-// VTTCue is currently the only TextTrackCue subclass.
-DEFINE_TYPE_CASTS(VTTCue, TextTrackCue, cue, true, true);
+DEFINE_TYPE_CASTS(VTTCue, TextTrackCue, cue, cue->cueType() == TextTrackCue::WebVTT, cue.cueType() == TextTrackCue::WebVTT);
 
 } // namespace WebCore