Legacy access to viewer role.

        DEPRECATED:  use ``policy["roles/viewers"]`` instead

def viewers(self):
        """Legacy access to viewer role.

        DEPRECATED:  use ``policy["roles/viewers"]`` instead
        """
        result = set()
        for role in self._VIEWER_ROLES:
            for member in self._bindings.get(role, ()):
                result.add(member)
        return frozenset(result)