Migrate from std::string to tensorflow::tstring. (#7830)
Migrate from std::string to tensorflow::tstring. (#7830)

Note that during the transition period tstring is typedef'ed to
std::string.

See: https://github.com/tensorflow/community/pull/91
alexgorban/models,tombstone/models,alexgorban/models,alexgorban/models,tombstone/models,alexgorban/models,tombstone/models,tombstone/models,tombstone/models,tombstone/models,alexgorban/models