added SkSL bench

Bug: skia:
Change-Id: I8497ddbc5c5fef4e0fa87f3b9828034ba34284cc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217636
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
2 files changed