Use the standard downcasting macros for downcasting TextTrack objects.

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

 /*
  * Copyright (C) 2011 Google 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 10 matching lines @@
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (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 LoadableTextTrack_h
 #define LoadableTextTrack_h
 
 #include "core/html/track/TextTrack.h"
 #include "platform/heap/Handle.h"
-#include "wtf/PassRefPtr.h"
+#include "wtf/Assertions.h"
 
 namespace blink {
 
 class HTMLTrackElement;
 
 class LoadableTextTrack final : public TextTrack {
  public:
   static LoadableTextTrack* create(HTMLTrackElement* track) {
     return new LoadableTextTrack(track);
   }
@@ skipping 11 matching lines @@
   bool isDefault() const override;
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   explicit LoadableTextTrack(HTMLTrackElement*);
 
   Member<HTMLTrackElement> m_trackElement;
 };
 
+DEFINE_TYPE_CASTS(LoadableTextTrack,
+                  TextTrack,
+                  track,
+                  track->trackType() == TextTrack::TrackElement,
+                  track.trackType() == TextTrack::TrackElement);
+
 }  // namespace blink
 
 #endif  // LoadableTextTrack_h