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/tests/CanvasTest.cpp b/tests/CanvasTest.cpp
index 2be83f3..c062e6c 100644
--- a/tests/CanvasTest.cpp
+++ b/tests/CanvasTest.cpp
@@ -384,7 +384,7 @@
     paint.setShader(SkShader::MakeBitmapShader(d.fBitmap, SkShader::kClamp_TileMode,
                                                SkShader::kClamp_TileMode));
     canvas->drawVertices(SkCanvas::kTriangleFan_VertexMode, 4, pts, pts,
-                         nullptr, nullptr, nullptr, 0, paint);
+                         nullptr, SkBlendMode::kModulate, nullptr, 0, paint);
 }
 // NYI: issue 240.
 TEST_STEP_NO_PDF(DrawVerticesShader, DrawVerticesShaderTestStep);