Simplify the signature of sk/aaa_fill_path

Previously, the clipRect is either equal to nullptr or clipRgn's bound
(after necessary supersampling shift). Hence we drop one of them to make
the signature simpler.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4874

Change-Id: Ied8d5313809d6cf90374365b01f2b8d52f2236e2
Reviewed-on: https://skia-review.googlesource.com/4874
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>