getStartTime(contentType) {
    const timescale = shaka.util.TsParser.Timescale;
    if (contentType == 'audio') {
      let audioStartTime = null;
      const audioData = this.getAudioData();
      if (audioData.length) {
        const pes = audioData[0];
        audioStartTime = Math.min(pes.dts, pes.pts) / timescale;
      }
      return audioStartTime;
    } else if (contentType == 'video') {
      let videoStartTime = null;
      const videoData = this.getVideoData(/* naluProcessing= */ false);
      if (videoData.length) {
        const pes = videoData[0];
        videoStartTime = Math.min(pes.dts, pes.pts) / timescale;
      }
      return videoStartTime;
    }
    return null;
  }
Return the start time for the audio and video

@param {string} contentType
@return {?number}
@export