message: Enable tensorflow::tstring stand-in class.
  After this CL, tensorflow::tstring will no longer be aliased to std::string.
diff: @@ -20,7 +20,7 @@ limitations under the License.
  #include <string>
  // TODO(b/138799229): Used to toggle until global presubmits pass.
  -// #define USE_TSTRING
  +#define USE_TSTRING
  #ifdef USE_TSTRING
  