message: Migrate from std::string to tensorflow::tstring.
  Note that during the transition period tstring is typedef'ed to std::string.
  This is a part of a larger migration effort for tensorflow::tstring.
  See:
diff: @@ -346,10 +346,10 @@ StatusOr<std::vector<HloSnapshot>> ParseRecordIoFile(absl::string_view filename,
  std::vector<HloSnapshot> snapshots;
  uint64 offset = 0;
  - string record;
  + tensorflow::tstring record;
  while (reader.ReadRecord(&offset, &record).ok()) {
  HloSnapshot snapshot;
  - if (snapshot.mutable_hlo()->ParseFromString(record)) {
  + if (snapshot.mutable_hlo()->ParseFromStringPiece(record)) {
  snapshots.push_back(std::move(snapshot));
  } else {
  LOG(ERROR) << "Encountered bad proto";
  