unify peekPixels around pixmap parameter

requires this chrome CL to land first
https://codereview.chromium.org/1775393003/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1784563002

Review URL: https://codereview.chromium.org/1784563002
17 files changed