add alias for xfermode::getproc to allow legacy g client to bulid

BUG=skia:
NOTRY=True

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4623

Change-Id: Ib8108c6e2f2ca4592428711c2ae8a0bd1fa7963d
Reviewed-on: https://skia-review.googlesource.com/4623
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>