Make path in Sk1DPathEffect thread safe.

Found by TSAN reporting on a recorded Sk1DPathEffect being drawn from
multiple threads.

Change-Id: I6d4bf9c336d9fd091263babd558891e64203e281
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207180
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed