Make GrReducedClip's gen ID only apply to the element list

Renames fGenID to fElementsGenID and designates this value as
undefined when when the element list is empty.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2244223004

Review-Url: https://codereview.chromium.org/2244223004
4 files changed