add bench for mixers

Bug: skia:
Change-Id: I1b12c0deac802fb9da8bd285c1ff6d7e63da00f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201687
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
2 files changed