Implemented as a different subclass for SkColorShader (which is also private) partly to make the CL clearer/simpler, and partly for flatten/unflatten compatibility. Later I'm sure we could combine these if that proves useful.
Lots of TODOs at the moment, but still valuable to get reviewed.
Note: this ignores the question (for the moment) about how to interpret SkColor in the larger world. That needs to happen, but this CL is more focused on what *else* to do besides handle the old-style input (and old-style pipeline).
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1934313002
Review-Url: https://codereview.chromium.org/1934313002
