Add skstd::optional::emplace.

Change-Id: I178ae881191e8ed7ce5399e806838a44d2358e31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/442319
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed