load_(opt_callback) {
    var getTx = this.db_.createTransaction(
        [e2e.ext.utils.IndexedDbStorage.OBJECTSTORE_NAME_]);
    var request =
        getTx.objectStore(e2e.ext.utils.IndexedDbStorage.OBJECTSTORE_NAME_)
            .get(this.storageKey_);
    request.addCallback(function(result) {
      this.storage_ = result || {};
      opt_callback && opt_callback(this);
    }, this);
  }
Initializes internal storage with the data in IndexedDB.
@param {function(!e2e.ext.utils.IndexedDbStorage)=} opt_callback
@private