getCell(mechanismName, cellName) {
    const mechanism = this.mechanisms_.get(mechanismName);
    if (!mechanism) {
      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 mechanism with name ' + mechanismName);
    }

    const cell = mechanism.getCells().get(cellName);
    if (!cell) {
      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 cell with name ' + cellName);
    }

    return cell;
  }
Get a specific storage cell. The promise will resolve with the storage
cell if it is found. If the storage cell is not found, the promise will
be rejected.

@param {string} mechanismName
@param {string} cellName
@return {!shaka.extern.StorageCell}