static isEmbeddedText_(mediaState) {
    const MimeUtils = shaka.util.MimeUtils;
    const CEA608_MIME = MimeUtils.CEA608_CLOSED_CAPTION_MIMETYPE;
    const CEA708_MIME = MimeUtils.CEA708_CLOSED_CAPTION_MIMETYPE;
    return mediaState &&
        mediaState.type == shaka.util.ManifestParserUtils.ContentType.TEXT &&
        (mediaState.stream.mimeType == CEA608_MIME ||
         mediaState.stream.mimeType == CEA708_MIME);
  }
@param {shaka.media.StreamingEngine.MediaState_} mediaState
@return {boolean}
@private