docs: def GrantApproval(self, requestor_username, approval_id, grantor_username):
      """Grants approval for a given request using given username."""
      try:
        approval = self.approvals_by_username[requestor_username][approval_id]
        approval.grants.append(
            rdf_objects.ApprovalGrant(
                grantor_username=grantor_username,
                timestamp=rdfvalue.RDFDatetime.Now()))
      except KeyError:
        raise db.UnknownApprovalRequestError("Can't find approval with id: %s" %
                                             approval_id)