message: Enforce xla in xla_control_flow_ops_test.
  I actually don't understand how one can avoid using xla in an xla
  test. But the test seems to pass now.
diff: @@ -233,8 +233,7 @@ class WhileV2Test(PForTestCase):
  body,
  [True, 0, 0.])
  - # b/155430349: Enabling forrce_xla=True triggers a CHECK in debug mode.
  - self._test_loop_fn(loop_fn, 3, force_xla=False)
  + self._test_loop_fn(loop_fn, 3, force_xla=True)
  if __name__ == "__main__":
  