Use SkMutex in SkSL.

Change-Id: I1b14e4c8de0ca60a0ebbdbc225befc5074e8de22
Bug: skia:10862
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329177
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed