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/samplecode/SampleFilterFuzz.cpp b/samplecode/SampleFilterFuzz.cpp
index 28aa5f4..139166f 100644
--- a/samplecode/SampleFilterFuzz.cpp
+++ b/samplecode/SampleFilterFuzz.cpp
@@ -558,7 +558,7 @@
     case MERGE:
         filter = SkMergeImageFilter::Make(make_image_filter(),
                                           make_image_filter(),
-                                          (SkXfermode::Mode)make_xfermode());
+                                          make_xfermode());
         break;
     case COLOR: {
         sk_sp<SkColorFilter> cf(make_color_filter());