add bench for computing convexity

Bug: skia:
Change-Id: Iaa2971c28f6a8e869b92887e0c0595c73b0aebdb
Reviewed-on: https://skia-review.googlesource.com/c/173225
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed