switch xfermodes over to sk_sp

src/utils/SkLua.cpp

Index: src/utils/SkLua.cpp
diff --git a/src/utils/SkLua.cpp b/src/utils/SkLua.cpp
index 01bf507f170052504bb40206434bb64339c43e33..9af5bfd84e6fe38bced97df729a7eb113c032f65 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -1098,7 +1098,7 @@ static int lpaint_getXfermode(lua_State* L) {
 
 static int lpaint_setXfermode(lua_State* L) {
     SkPaint* paint = get_obj<SkPaint>(L, 1);
-    paint->setXfermode(get_ref<SkXfermode>(L, 2));
+    paint->setXfermode(sk_ref_sp(get_ref<SkXfermode>(L, 2)));
     return 0;
 }