change SkXfermodeImageFilter to carry no impl information

BUG=skia:

NOTREECHECKS=True

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2584

Change-Id: Ibf2dfa7722348ae71a99f1c895de0d2996350e58
Reviewed-on: https://skia-review.googlesource.com/2584
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed