Add tensorflow::gtl::optional.

Change: 149674904

tensorflow/core/BUILD

@@ -250,6 +250,7 @@ cc_library(
         "lib/gtl/flatmap.h",
         "lib/gtl/flatset.h",
         "lib/gtl/inlined_vector.h",
+        "lib/gtl/optional.h",
         "lib/gtl/priority_queue_util.h",
         "lib/hash/crc32c.h",
         "lib/histogram/histogram.h",
@@ -1710,6 +1711,7 @@ tf_cc_tests(
         "lib/gtl/iterator_range_test.cc",
         "lib/gtl/manual_constructor_test.cc",
         "lib/gtl/map_util_test.cc",
+        "lib/gtl/optional_test.cc",
         "lib/gtl/top_n_test.cc",
         "lib/hash/crc32c_test.cc",
         "lib/hash/hash_test.cc",