remove xfermode from public api

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4020

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: I19cd056f2af778f10e8c6c2b7b2735593b43dbac
Reviewed-on: https://skia-review.googlesource.com/4020
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gm/drawatlas.cpp b/gm/drawatlas.cpp
index 43b103c..c098e15 100644
--- a/gm/drawatlas.cpp
+++ b/gm/drawatlas.cpp
@@ -89,7 +89,7 @@
 
         canvas->drawAtlas(atlas.get(), xform, tex, N, nullptr, &paint);
         canvas->translate(0, 100);
-        canvas->drawAtlas(atlas.get(), xform, tex, colors, N, SkXfermode::kSrcIn_Mode, nullptr, &paint);
+        canvas->drawAtlas(atlas.get(), xform, tex, colors, N, SkBlendMode::kSrcIn, nullptr, &paint);
     }
 
 private: