Add nullopt to skstd::optional.

Change-Id: Ie0ed256fce79e2a03fcd468c820f468dc8bf25b5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/420536
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
2 files changed