message: Properly serialize ResourceVariable global_step into the metagraph.
  Prior to this, saving and restoring a graph with a resource variable global_step
  would cause the global_step collection of the reimported graph to contain
  a resource tensor (the object underlying the ResourceVariable); the actual
  metadata associated with it would be serialized.
diff: @@ -1087,6 +1087,11 @@ ops.register_proto_function(
  proto_type=variable_pb2.VariableDef,
  to_proto=_to_proto_fn,
  from_proto=_from_proto_fn)
  +ops.register_proto_function(
  + ops.GraphKeys.GLOBAL_STEP,
  + proto_type=variable_pb2.VariableDef,
  + to_proto=_to_proto_fn,
  + from_proto=_from_proto_fn)
  def is_resource_variable(var):
  