frma_(codecs) {
    const codec = codecs.split('.')[0];
    const Mp4Generator = shaka.util.Mp4Generator;
    const codecNumber = this.stringToCharCode_(codec);
    const bytes = new Uint8Array([
      ...this.breakNumberIntoBytes_(codecNumber, 4),
    ]);
    return Mp4Generator.box('frma', bytes);
  }
Generate a FRMA box

@param {string} codecs
@return {!Uint8Array}
@private