Use the standard downcasting macros for downcasting TextTrack objects.

third_party/WebKit/Source/core/html/track/InbandTextTrack.h

 /*
  * 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:
  * 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 11 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef InbandTextTrack_h
 #define InbandTextTrack_h
 
 #include "core/html/track/TextTrack.h"
 #include "platform/heap/Handle.h"
 #include "public/platform/WebInbandTextTrackClient.h"
-#include "wtf/RefPtr.h"
+#include "wtf/Assertions.h"
 
 namespace blink {
 class WebInbandTextTrack;
 class WebString;
 }
 
 namespace blink {
 
 class InbandTextTrack final : public TextTrack,
                               public WebInbandTextTrackClient {
  public:
   static InbandTextTrack* create(WebInbandTextTrack*);
   ~InbandTextTrack() override;
 
   void setTrackList(TextTrackList*) override;
 
  private:
-  InbandTextTrack(WebInbandTextTrack*);
+  explicit InbandTextTrack(WebInbandTextTrack*);
 
   void addWebVTTCue(double,
                     double,
                     const WebString&,
                     const WebString&,
                     const WebString&) override;
 
   WebInbandTextTrack* m_webTrack;
 };
 
+// All concrete implementations of WebInbandTextTrackClient are
+// InbandTextTracks.
+DEFINE_TYPE_CASTS(InbandTextTrack, WebInbandTextTrackClient, track, true, true);
+
 }  // namespace blink
 
 #endif