code: private String trackKindToString(final Track.Kind kind) {
  		if (kind == null) {
  			return null;
  		}
  
  		switch (kind) {
  			case SUBTITLES:
  				return "subtitles";
  
  			case CAPTIONS:
  				return "captions";
  
  			case DESCRIPTIONS:
  				return "descriptions";
  
  			case CHAPTERS:
  				return "chapters";
  
  			case METADATA:
  				return "metadata";
  
  			default:
  				LOG.error("Unknown track kind " + kind);
  				return null;
  		}
  	}
docs: Converts a Track kind to the client track kind identifier.
  
  @param kind the Track Kind to convert.
  @return a client track kind identifier, or null if it could not be converted.