Bundle SkShader::asFragmentProcessor arguments in a struct

The signature of this thing keeps changing (and is about to change again).
This just makes maintenance much easier.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2175563003

Review-Url: https://codereview.chromium.org/2175563003
34 files changed