Use the standard downcasting macros for downcasting TextTrack objects.

third_party/WebKit/Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 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
@@ skipping 2669 matching lines @@
   // cancelable, and that uses the TrackEvent interface, with the track
   // attribute initialized to the text track's TextTrack object, at the media
   // element's textTracks attribute's TextTrackList object.
   textTracks()->append(textTrack);
 }
 
 void HTMLMediaElement::removeTextTrack(WebInbandTextTrack* webTrack) {
   if (!m_textTracks)
     return;
 
-  // This cast is safe because we created the InbandTextTrack with the
-  // WebInbandTextTrack passed to mediaPlayerDidAddTextTrack.
-  InbandTextTrack* textTrack =
-      static_cast<InbandTextTrack*>(webTrack->client());
+  // This cast is safe because InbandTextTrack is the only concrete
+  // implementation of WebInbandTextTrackClient.
+  InbandTextTrack* textTrack = toInbandTextTrack(webTrack->client());
   if (!textTrack)
     return;
 
   m_textTracks->remove(textTrack);
 }
 
 void HTMLMediaElement::forgetResourceSpecificTracks() {
   // Implements the "forget the media element's media-resource-specific tracks"
   // algorithm.  The order is explicitly specified as text, then audio, and
   // finally video.  Also 'removetrack' events should not be fired.
@@ skipping 1427 matching lines @@
       kMostlyFillViewportBecomeStableSeconds, BLINK_FROM_HERE);
 }
 
 void HTMLMediaElement::viewportFillDebouncerTimerFired(TimerBase*) {
   m_mostlyFillingViewport = true;
   if (m_webMediaPlayer)
     m_webMediaPlayer->becameDominantVisibleContent(m_mostlyFillingViewport);
 }
 
 }  // namespace blink