Implement DataCue interface.

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

 /*
  * Copyright (c) 2013, Opera Software ASA. 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 116 matching lines @@
     enum CueAlignment {
         Start = 0,
         Middle,
         End,
         Left,
         Right,
         NumberOfAlignments
     };
     CueAlignment getAlignment() const { return m_cueAlignment; }
 
+    virtual CueType cueType() const OVERRIDE { return WebVTT; }
+
     virtual ExecutionContext* executionContext() const OVERRIDE;
 
 #ifndef NDEBUG
     virtual String toString() const OVERRIDE;
 #endif
 
     virtual void trace(Visitor*) OVERRIDE;
 
 private:
     VTTCue(Document&, double startTime, double endTime, const String& text);
@@ skipping 38 matching lines @@
 
     CSSValueID m_displayDirection;
     int m_displaySize;
     std::pair<float, float> m_displayPosition;
 
     bool m_snapToLines : 1;
     bool m_displayTreeShouldChange : 1;
     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
 
 #endif