Encapsulate GrReducedClip result in class members

Updates GrReducedClip to store its result in class members instead of
various pointer arguments. This helps clean up calling code and will
make it easier to reduce the clip higher in the stack.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2222873002

Review-Url: https://codereview.chromium.org/2222873002
7 files changed