message: Updated tools/ to use tstring.
  This is a part of a larger migration effort for tensorflow::tstring.
  See:
diff: @@ -107,7 +107,7 @@ class SparsifyGatherTest : public ::testing::Test {
  CreateNode("save/Const", "Const", {}, &graph_def);
  Tensor tensor_names_values(DT_STRING, TensorShape({1}));
  - test::FillValues<string>(&tensor_names_values, {"w"});
  + test::FillValues<tstring>(&tensor_names_values, {"w"});
  NodeDef* tensor_names_node =
  CreateNode("save/RestoreV2/tensor_names", "Const", {}, &graph_def);
  SetNodeTensorAttr<string>("value", tensor_names_values,
  @@ -320,7 +320,7 @@ class SparsifyGatherTest : public ::testing::Test {
  NodeDef* tensor_names_node =
  CreateNode("save/RestoreV2/tensor_names", "Const", {}, &graph_def);
  Tensor tensor_names_values(DT_STRING, TensorShape({2}));
  - test::FillValues<string>(&tensor_names_values, {"w1", "w2"});
  + test::FillValues<tstring>(&tensor_names_values, {"w1", "w2"});
  SetNodeTensorAttr<string>("value", tensor_names_values,
  tensor_names_node);
  