Use the standard downcasting macros for downcasting TextTrack objects.

third_party/WebKit/Source/core/html/track/CueTimeline.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/html/track/CueTimeline.h"
 
 #include "core/events/Event.h"
 #include "core/html/HTMLMediaElement.h"
 #include "core/html/HTMLTrackElement.h"
 #include "core/html/track/LoadableTextTrack.h"
@@ skipping 309 matching lines @@
   // task to fire a simple event named cuechange at the TextTrack object, and,
   // ...
   for (const auto& track : affectedTracks) {
     mediaElement.scheduleEvent(
         createEventWithTarget(EventTypeNames::cuechange, track.get()));
 
     // ... if the text track has a corresponding track element, to then fire a
     // simple event named cuechange at the track element as well.
     if (track->trackType() == TextTrack::TrackElement) {
       HTMLTrackElement* trackElement =
-          static_cast<LoadableTextTrack*>(track.get())->trackElement();
+          toLoadableTextTrack(track.get())->trackElement();
       DCHECK(trackElement);
       mediaElement.scheduleEvent(
           createEventWithTarget(EventTypeNames::cuechange, trackElement));
     }
   }
 
   // 16 - Set the text track cue active flag of all the cues in the current
   // cues, and unset the text track cue active flag of all the cues in the
   // other cues.
   for (const auto& cue : currentCues)
@@ skipping 21 matching lines @@
   --m_ignoreUpdate;
   if (!m_ignoreUpdate)
     updateActiveCues(mediaElement().currentTime());
 }
 
 DEFINE_TRACE(CueTimeline) {
   visitor->trace(m_mediaElement);
 }
 
 }  // namespace blink