forEachCell(callback) {
    this.mechanisms_.forEach((mechanism, mechanismName) => {
      mechanism.getCells().forEach((cell, cellName) => {
        const path = {
          mechanism: mechanismName,
          cell: cellName,
        };

        callback(path, cell);
      });
    });
  }
@param {function(!shaka.offline.StorageCellPath,
                 !shaka.extern.StorageCell)} callback