constructor (videoTrack, audioTrack, metadataTrack) {
    this.videoTrack = videoTrack || new VideoTrack()
    this.audioTrack = audioTrack || new AudioTrack()
    this.metadataTrack = metadataTrack || new MetadataTrack()
    this._fixer = new FlvFixer(this.videoTrack, this.audioTrack, this.metadataTrack)
  }
@param {VideoTrack} [videoTrack]
@param {AudioTrack} [audioTrack]
@param {MetadataTrack} [metadataTrack]