returnStorage = function() {
    return storage;
  }
Returns a result that will be resolved with the LockableStorage when the
underlying storage has been read from. Use this to avoid race conditions that
the constructor may introduce.
@param {!goog.storage.mechanism.Mechanism} storageMechanism persistent
   storage mechanism.
@param {function(): !e2e.async.Result<string>=} opt_passphraseCallback
   Callback used to deliver the passphrase.
@param {string=} opt_storageKeyName The storage's key under which the user
   data is stored in storageMechanism.
@param {string=} opt_saltKeyName The storage's key under which the salt is
   stored in storageMechanism.
@return {!e2e.async.Result<!e2e.openpgp.LockableStorage>} The storage ready
to be used (check if it needs unlocking with isLocked()).