MSE: Implement TrackDefault object

Source/core/html/track/TextTrack.cpp

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 107 matching lines @@
         for (size_t i = 0; i < m_cues->length(); ++i)
             m_cues->item(i)->setTrack(0);
     }
     if (m_regions) {
         for (size_t i = 0; i < m_regions->length(); ++i)
             m_regions->item(i)->setTrack(0);
     }
 #endif
 }
 
-bool TextTrack::isValidKindKeyword(const AtomicString& value)
+bool TextTrack::isValidKindKeyword(const String& value)
 {
     if (value == subtitlesKeyword())
         return true;
     if (value == captionsKeyword())
         return true;
     if (value == descriptionsKeyword())
         return true;
     if (value == chaptersKeyword())
         return true;
     if (value == metadataKeyword())
@@ skipping 301 matching lines @@
 void TextTrack::trace(Visitor* visitor)
 {
     visitor->trace(m_cues);
     visitor->trace(m_regions);
     visitor->trace(m_trackList);
     TrackBase::trace(visitor);
     EventTargetWithInlineData::trace(visitor);
 }
 
 } // namespace blink