PathOpSubmitter: switch from using a Mutex to using SkOnce

Change-Id: I6e014afe71d056912a2afb2adf18cf517cd87364
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565910
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed