determineStartTime_(playlist) {
    // If we already have a starttime we avoid processing this again.
    if (this.startTime_ != null) {
      return;
    }
    const startTimeTag =
        shaka.hls.Utils.getFirstTagWithName(playlist.tags, 'EXT-X-START');
    if (startTimeTag) {
      this.startTime_ =
          Number(startTimeTag.getRequiredAttrValue('TIME-OFFSET'));
    }
  }
@param {!shaka.hls.Playlist} playlist
@private