static convertCodecs(contentType, mimeType) {
    const TransmuxerEngine = shaka.transmuxer.TransmuxerEngine;
    const transmuxerPlugin = TransmuxerEngine.findTransmuxer(mimeType);
    if (!transmuxerPlugin) {
      return mimeType;
    }
    const transmuxer = transmuxerPlugin();
    const codecs = transmuxer.convertCodecs(contentType, mimeType);
    transmuxer.destroy();
    return codecs;
  }
For any stream, convert its codecs to MP4 codecs.
@param {string} contentType
@param {string} mimeType
@return {string}