Revert "replace SkXfermode obj with SkBlendMode enum in paints"

This reverts commit I4fb489ba6b3f77b458f7e4a99f79c7ad10859135.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> 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>
> 

TBR=bsalomon@google.com,fmalita@chromium.org,fmalita@google.com,reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I3e43f79ef5c1709929663fe63cc1f67cd78270b7
Reviewed-on: https://skia-review.googlesource.com/2871
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
139 files changed