Round to nearest in ScalarTo256.

This is a followup to https://codereview.chromium.org/1693683002 per discussion on https://codereview.chromium.org/1691403002.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1723483004

Review URL: https://codereview.chromium.org/1723483004
1 file changed