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/SkLua.cpp b/src/utils/SkLua.cpp
index ba311af..14f074c 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -573,7 +573,7 @@
         texs = texStorage;
     }
 
-    get_ref<SkCanvas>(L, 1)->drawPatch(cubics, colors, texs, nullptr, *get_obj<SkPaint>(L, 5));
+    get_ref<SkCanvas>(L, 1)->drawPatch(cubics, colors, texs, *get_obj<SkPaint>(L, 5));
     return 0;
 }