startOperation_(store, type) {
    const transaction = this.connection_.transaction([store], type);
    const operation =
        new shaka.offline.indexeddb.DBOperation(transaction, store);

    this.pending_.push(operation);

    // Once the operation is done (regardless of outcome) stop tracking it.
    operation.promise().then(
        () => this.stopTracking_(operation),
        () => this.stopTracking_(operation));

    return operation;
  }
@param {string} store The name of the store that the operation should
                      occur on.
@param {string} type The type of operation being performed on the store.
                     This determines what commands may be performed. This
                     can either be "readonly" or "readwrite".
@return {!shaka.offline.indexeddb.DBOperation}
@private