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/src/utils/SkLuaCanvas.cpp b/src/utils/SkLuaCanvas.cpp
index 6b769f0..10fc3b4 100644
--- a/src/utils/SkLuaCanvas.cpp
+++ b/src/utils/SkLuaCanvas.cpp
@@ -309,7 +309,7 @@
 
 void SkLuaCanvas::onDrawVertices(VertexMode vmode, int vertexCount,
                                  const SkPoint vertices[], const SkPoint texs[],
-                                 const SkColor colors[], SkXfermode* xmode,
+                                 const SkColor colors[], SK_XFERMODE_PARAM,
                                  const uint16_t indices[], int indexCount,
                                  const SkPaint& paint) {
     AUTO_LUA("drawVertices");