This is going to be faster, and it's the last usage of GrColorMul
Bug: skia:
Change-Id: I2ecbed55ec401792f8d8f8ab9dc91a14182094b4
Reviewed-on: https://skia-review.googlesource.com/c/171520
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>