getActive() {
    /** @type {?shaka.offline.StorageCellHandle} */
    let handle = null;

    this.mechanisms_.forEach((mechanism, mechanismName) => {
      mechanism.getCells().forEach((cell, cellName) => {
        // If this cell is not useful to us or we already have a handle, then
        // we don't need to make a new handle.
        if (cell.hasFixedKeySpace() || handle) {
          return;
        }

        const path = {
          mechanism: mechanismName,
          cell: cellName,
        };

        handle = {
          path: path,
          cell: cell,
        };
      });
    });

    if (handle) {
      return /** @type {shaka.offline.StorageCellHandle} */(handle);
    }

    throw new shaka.util.Error(
        shaka.util.Error.Severity.CRITICAL,
        shaka.util.Error.Category.STORAGE,
        shaka.util.Error.Code.MISSING_STORAGE_CELL,
        'Could not find a cell that supports add-operations');
  }
Get a promise that will resolve with a storage cell that supports
add-operations. If no cell can be found, the promise will be rejected.

@return {shaka.offline.StorageCellHandle}