API change to allow for NormalSource selection at the user level.

This CL's base is the CL for CPU handling: https://codereview.chromium.org/2050773002/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2063793002

Review-Url: https://codereview.chromium.org/2063793002
8 files changed