replace SkXfermode obj with SkBlendMode enum in paints

BUG=skia:5814

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2714

Change-Id: I4fb489ba6b3f77b458f7e4a99f79c7ad10859135
Reviewed-on: https://skia-review.googlesource.com/2714
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
139 files changed