[Video] Add dummy video/audio tracks inactive to select them later.

third_party/WebKit/Source/modules/mediasource/MediaSource.cpp

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 447 matching lines @@
     return TimeRanges::create(
         0, buffered->end(buffered->length() - 1, ASSERT_NO_EXCEPTION));
   }
 
   // 3. Otherwise: Return a single range with a start time of 0 and an end time
   //    equal to duration.
   return TimeRanges::create(0, sourceDuration);
 }
 
 void MediaSource::onTrackChanged(TrackBase* track) {
-  DCHECK(RuntimeEnabledFeatures::audioVideoTracksEnabled());
+  DCHECK(HTMLMediaElement::mediaTracksEnabledInternally());
   SourceBuffer* sourceBuffer =
       SourceBufferTrackBaseSupplement::sourceBuffer(*track);
   if (!sourceBuffer)
     return;
 
   DCHECK(m_sourceBuffers->contains(sourceBuffer));
   if (track->type() == WebMediaPlayer::AudioTrack) {
     sourceBuffer->audioTracks().scheduleChangeEvent();
   } else if (track->type() == WebMediaPlayer::VideoTrack) {
     if (static_cast<VideoTrack*>(track)->selected())
@@ skipping 336 matching lines @@
   event->setTarget(this);
 
   m_asyncEventQueue->enqueueEvent(event);
 }
 
 URLRegistry& MediaSource::registry() const {
   return MediaSourceRegistry::registry();
 }
 
 }  // namespace blink