don't memtion SkXfermode in public

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5070

Change-Id: Ia9527bfd0f13146669df5e71098af903e14bae3a
Reviewed-on: https://skia-review.googlesource.com/5070
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
9 files changed