Bundle common arguments to GrGradientEffect creation

I'm going to be adding more (to deal with color spaces), so trying to fix
this before it gets out of control.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2332213007

Review-Url: https://codereview.chromium.org/2332213007
8 files changed