expandRobustness = (drmInfos, robustnessType) => {
      const newDrmInfos = [];
      for (const drmInfo of drmInfos) {
        let items = drmInfo[robustnessType] ||
            (advanced.has(drmInfo.keySystem) &&
            advanced.get(drmInfo.keySystem)[robustnessType]) || '';
        if (items == '' &&
            shaka.drm.DrmUtils.isWidevineKeySystem(drmInfo.keySystem)) {
          if (robustnessType == 'audioRobustness') {
            items = [this.config_.defaultAudioRobustnessForWidevine];
          } else if (robustnessType == 'videoRobustness') {
            items = [this.config_.defaultVideoRobustnessForWidevine];
          }
        }
        if (typeof items === 'string') {
          // if drmInfo's robustness has already been expanded,
          // use the drmInfo directly.
          newDrmInfos.push(drmInfo);
        } else if (Array.isArray(items)) {
          if (items.length === 0) {
            items = [''];
          }
          for (const item of items) {
            newDrmInfos.push(
                Object.assign({}, drmInfo, {[robustnessType]: item}),
            );
          }
        }
      }
      return newDrmInfos;
    }
Expand robustness into multiple drm infos if multiple video robustness
levels were provided.

robustness can be either a single item as a string or multiple items as
an array of strings.

@param {!Array<shaka.extern.DrmInfo>} drmInfos
@param {string} robustnessType
@return {!Array<shaka.extern.DrmInfo>}