Revert[5] "replace SkXfermode obj with SkBlendMode enum in paints"

Revert[5] "replace SkXfermode obj with SkBlendMode enum in paints"

This reverts commit I0fa5c58af428f3da8565465d1219a34ef8417d9a.

Reason for revert: failing to deserialize some of the 100K

Original change's description:
> Revert[4] "replace SkXfermode obj with SkBlendMode enum in paints"
>
> This reverts commit 2cbcd12281ee807214df094964c584c78932e10b.
>
> BUG=skia:
>
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2924
>
> Change-Id: I0fa5c58af428f3da8565465d1219a34ef8417d9a
> Reviewed-on: https://skia-review.googlesource.com/2924
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Mike Reed <reed@google.com>
>

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

Change-Id: I1805a57eef5ebcac203da5989c8539345ecf806f
Reviewed-on: https://skia-review.googlesource.com/2962
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>