Grants approval for a given request using given username.

def GrantApproval(self,
                    requestor_username,
                    approval_id,
                    grantor_username,
                    cursor=None):
    """Grants approval for a given request using given username."""
    self._GrantApproval(
        requestor_username, _ApprovalIDToInt(approval_id), grantor_username,
        cursor)