sketch SkValue API

I think we may be trying to take too many steps at once.
Let's try starting from an API and evolving from there?

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1589953003

Review URL: https://codereview.chromium.org/1589953003