resolvePath(path) {
    const mechanism = this.mechanisms_.get(path.mechanism);

    if (!mechanism) {
      return null;
    }

    return mechanism.getCells().get(path.cell);
  }
Find the cell that the path points to. A path is made up of a mount point
and a cell id. If a cell can be found, the cell will be returned. If no
cell is found, null will be returned.

@param {shaka.offline.StorageCellPath} path
@return {shaka.extern.StorageCell}