Update SkAlphaThresholdFilter to sk_sp

TBR=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1847053004

Review URL: https://codereview.chromium.org/1847053004
diff --git a/samplecode/SampleFilterFuzz.cpp b/samplecode/SampleFilterFuzz.cpp
index 3b31ef7..dd17e7e 100644
--- a/samplecode/SampleFilterFuzz.cpp
+++ b/samplecode/SampleFilterFuzz.cpp
@@ -555,9 +555,10 @@
 
     switch (R(NUM_FILTERS)) {
     case ALPHA_THRESHOLD:
-        filter = sk_sp<SkImageFilter>(SkAlphaThresholdFilter::Create(make_region(),
-                                                                     make_scalar(),
-                                                                     make_scalar()));
+        filter = SkAlphaThresholdFilter::Make(make_region(),
+                                              make_scalar(),
+                                              make_scalar(),
+                                              nullptr);
         break;
     case MERGE:
         filter = SkMergeImageFilter::Make(make_image_filter(),