schm_(stream) {
    let encryptionScheme = 'cenc';
    const drmInfo = stream.drmInfos[0];
    if (drmInfo && drmInfo.encryptionScheme) {
      encryptionScheme = drmInfo.encryptionScheme;
    }
    const Mp4Generator = shaka.util.Mp4Generator;
    const schemeNumber = this.stringToCharCode_(encryptionScheme);
    const bytes = new Uint8Array([
      0x00, // version 0
      0x00, 0x00, 0x00, // flags
      ...this.breakNumberIntoBytes_(schemeNumber, 4), // Scheme
      0x00, 0x01, 0x00, 0x00, // Scheme version: 1.0
    ]);
    return Mp4Generator.box('schm', bytes);
  }
Generate a SCHM box

@param {!shaka.extern.Stream} stream
@return {!Uint8Array}
@private