Revert[7] "replace SkXfermode obj with SkBlendMode enum in paints"

This reverts commit Ib4a154cdd5f5d1dcac921ef50d53b79a2d6a1be8.

Reason for revert: new assert from 100K bot

Original change's description:
> Revert[6] "replace SkXfermode obj with SkBlendMode enum in paints"
> 
> - perform version check in CreateProc for XfermodeImageFilter and ArithmeticImageFilter
> This reverts commit 3ed485f4249e17abb4b11f5018d03175fd1afb44.
> 
> BUG=skia:
> 
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2992
> 
> Change-Id: Ib4a154cdd5f5d1dcac921ef50d53b79a2d6a1be8
> Reviewed-on: https://skia-review.googlesource.com/2992
> 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: I848e5a69c5cd67f2c14889f4f0a346652578c4ff
Reviewed-on: https://skia-review.googlesource.com/3023
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/gm/drawatlas.cpp b/gm/drawatlas.cpp
index 43b103c..dd81628 100644
--- a/gm/drawatlas.cpp
+++ b/gm/drawatlas.cpp
@@ -23,12 +23,12 @@
         canvas->clear(SK_ColorRED);
 
         SkPaint paint;
-        paint.setBlendMode(SkBlendMode::kClear);
+        paint.setXfermodeMode(SkXfermode::kClear_Mode);
         SkRect r(target);
         r.inset(-1, -1);
         // zero out a place (with a 1-pixel border) to land our drawing.
         canvas->drawRect(r, paint);
-        paint.setBlendMode(SkBlendMode::kSrcOver);
+        paint.setXfermode(nullptr);
         paint.setColor(SK_ColorBLUE);
         paint.setAntiAlias(true);
         canvas->drawOval(target, paint);