Revert[8] "replace SkXfermode obj with SkBlendMode enum in paints"

This reverts commit c245574ba3d0e2ade6c94b2812de3baa383bf4c4.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2396953002

TBR=

Review-Url: https://codereview.chromium.org/2396953002
diff --git a/gm/plus.cpp b/gm/plus.cpp
index 6a5af61..51ac2cb 100644
--- a/gm/plus.cpp
+++ b/gm/plus.cpp
@@ -37,7 +37,7 @@
 
     // Using Plus on the right should merge the AA of seam together completely covering the red.
     canvas->saveLayer(nullptr, nullptr);
-      p.setXfermodeMode(SkXfermode::kPlus_Mode);
+      p.setBlendMode(SkBlendMode::kPlus);
       canvas->translate(150, 0);
       canvas->drawPath(upperLeft, p);
       canvas->drawPath(bottomRight, p);