static createEmeSessionCell_(db) {
    const StorageMechanism = shaka.offline.indexeddb.StorageMechanism;
    const store = StorageMechanism.SESSION_ID_STORE;
    if (db.objectStoreNames.contains(store)) {
      shaka.log.debug('Mounting session ID idb storage cell');
      return new shaka.offline.indexeddb.EmeSessionStorageCell(db, store);
    }
    return null;
  }
@param {!IDBDatabase} db
@return {shaka.extern.EmeSessionStorageCell}
@private