addKeySystem(keySystem) {
    if (this.isClear()) {
      // Once an asset has an actual key system, it's no longer a CLEAR asset.
      this.drm = [];
    }
    this.drm.push(keySystem);
    // Sort the drm list, so that key systems are in a predictable order.
    this.drm.sort(ShakaDemoAssetInfo.caseLessAlphaComparator_);
    return this;
  }
@param {shakaAssets.KeySystem} keySystem
@return {!ShakaDemoAssetInfo}