Add BW masks to random scaler context

TBR=bsalomon@google.com
BUG=skia:

Committed: https://skia.googlesource.com/skia/+/a4a530614e9ba856aac3f16c29683e6a2a9153d9

Review URL: https://codereview.chromium.org/1270793003
3 files changed