Adds a new mutation for a key to be deleted.

        :rtype: :class:`.entity_pb2.Key`
        :returns: The newly created key protobuf that will be
                  deleted when sent with a commit.

def _add_delete_key_pb(self):
        """Adds a new mutation for a key to be deleted.

        :rtype: :class:`.entity_pb2.Key`
        :returns: The newly created key protobuf that will be
                  deleted when sent with a commit.
        """
        new_mutation = _datastore_pb2.Mutation()
        self._mutations.append(new_mutation)
        return new_mutation.delete