add unit test for clamp-before-premul

Bug: chromium:867813
Bug: chromium:862908

Change-Id: I79a82b6637c8a707844642b9bce4444bb85828b5
Reviewed-on: https://skia-review.googlesource.com/144295
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed