message: Fix a flaky test.
  In the old code, `a` can be freed before bytes_in_use, making the result of
  `bytes_in_use` unstable. In the new code, `b` uses `a` and control-depends on
  `bytes_in_use`, so `a` cannot be freed before `bytes_in_use`.
diff: @@ -76,9 +76,9 @@ class MemoryStatsOpsTest(test_util.TensorFlowTestCase):
  with ops.control_dependencies([a]):
  bytes_in_use_op = memory_stats_ops.BytesInUse()
  with ops.control_dependencies([bytes_in_use_op]):
  - b = random_ops.random_uniform(matrix_shape, dtype=dtype)
  + b = math_ops.add(a, a)
  - _, bytes_in_use, max_bytes_in_use = sess.run([a, bytes_in_use_op,
  + _, bytes_in_use, max_bytes_in_use = sess.run([b, bytes_in_use_op,
  max_bytes_in_use_op])
  # intermediate result allocates 1 matrix, max usage is at least 2
  