Implement gradient simplification for 0,0,1 and 0,1,1 gradients

Depends on https://codereview.chromium.org/2259823005/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2256843004

Review-Url: https://codereview.chromium.org/2256843004
1 file changed