Update SkMergeImageFilter to sk_sp

TBR=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1847583002

Review URL: https://codereview.chromium.org/1847583002
diff --git a/samplecode/SampleFilterFuzz.cpp b/samplecode/SampleFilterFuzz.cpp
index 103d8c4..a1e8c2d 100644
--- a/samplecode/SampleFilterFuzz.cpp
+++ b/samplecode/SampleFilterFuzz.cpp
@@ -559,14 +559,11 @@
                                                                      make_scalar(),
                                                                      make_scalar()));
         break;
-    case MERGE: {
-        sk_sp<SkImageFilter> subFilter1(make_image_filter());
-        sk_sp<SkImageFilter> subFilter2(make_image_filter());
-        filter = sk_sp<SkImageFilter>(SkMergeImageFilter::Create(subFilter1.get(), 
-                                                                 subFilter2.get(),
-                                                                 make_xfermode()));
+    case MERGE:
+        filter = SkMergeImageFilter::Make(make_image_filter(),
+                                          make_image_filter(),
+                                          make_xfermode());
         break;
-    }
     case COLOR: {
         sk_sp<SkColorFilter> cf(make_color_filter());
         sk_sp<SkImageFilter> subFilter(make_image_filter());