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/tests/FlattenableFactoryToName.cpp b/tests/FlattenableFactoryToName.cpp
index 28d491b..a22f0b1 100644
--- a/tests/FlattenableFactoryToName.cpp
+++ b/tests/FlattenableFactoryToName.cpp
@@ -29,8 +29,8 @@
     rects[1] = SkIRect::MakeXYWH(150, 0, 200, 500);
     SkRegion region;
     region.setRects(rects, 2);
-    SkAutoTUnref<SkImageFilter> filter( SkAlphaThresholdFilter::Create(region, 0.2f, 0.7f));
-    test_flattenable(r, filter, "SkAlphaThresholdFilter()");
+    sk_sp<SkImageFilter> filter(SkAlphaThresholdFilter::Make(region, 0.2f, 0.7f, nullptr));
+    test_flattenable(r, filter.get(), "SkAlphaThresholdFilter()");
 
     SkBitmap bm;
     bm.allocN32Pixels(8, 8);