use SkOnce for singleton patheffect

Change-Id: Iabeadca7dda29d0247536b5a02f977066772e2ee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/291657
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed