getType_(tag) {
    let type = tag.getRequiredAttrValue('TYPE').toLowerCase();
    if (type == 'subtitles') {
      type = shaka.util.ManifestParserUtils.ContentType.TEXT;
    }
    return type;
  }
Get the type value.
Shaka recognizes the content types 'audio', 'video', 'text', and 'image'.
The HLS 'subtitles' type needs to be mapped to 'text'.
@param {!shaka.hls.Tag} tag
@return {string}
@private