Combine duplicate code in preparing clipRgn for fill_path

Bug: skia:
Change-Id: I52ed98491457aa426e2bb74a29131f4a20330017
Reviewed-on: https://skia-review.googlesource.com/19341
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
4 files changed