ComparePixels helper takes GrCPixmaps

Change-Id: I56d902b652663df7893626d19bc88d18f980c02c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/391301
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed