Use native std::optional.

Change-Id: I3bcf7a23eb27e98d24840b492930955125d35bd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/501476
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed