progressCallback = (content, progress) => {
      const identifier = content.appMetadata['identifier'];
      const asset = getAssetWithIdentifier(identifier);
      if (asset) {
        asset.storedProgress = progress;
        this.dispatchEventWithName_('shaka-main-offline-progress');
      }
    }
@param {shaka.extern.StoredContent} content
@param {number} progress