addDrmInfo(keySystem, func) {
    goog.asserts.assert(this.manifest_,
        'A top-level generated Manifest is required to use this method!');

    const drmInfo =
        new shaka.test.ManifestGenerator.DrmInfo(this.manifest_, keySystem);

    if (func) {
      func(drmInfo);
    }

    if (!this.drmInfos) {
      // This may be the case if this was created through addPartialStream.
      this.drmInfos = [];
    }
    this.drmInfos.push(drmInfo.build_());
  }
Adds a new DrmInfo to the current Stream.

@param {string} keySystem
@param {function(!shaka.test.ManifestGenerator.DrmInfo)=} func