Update SkAlphaThresholdFilter to sk_sp

gm/imagealphathreshold.cpp

Index: gm/imagealphathreshold.cpp
diff --git a/gm/imagealphathreshold.cpp b/gm/imagealphathreshold.cpp
index 8d65cf8ee756d71cf1b51c0ff48967ed62baf054..fdd93bd2105cc1ffa5a02e2fc455be26e4e90ed5 100644
--- a/gm/imagealphathreshold.cpp
+++ b/gm/imagealphathreshold.cpp
@@ -35,7 +35,7 @@ SkPaint create_filter_paint() {
     region.setRects(rects, 2);
 
     SkPaint paint;
-    paint.setImageFilter(SkAlphaThresholdFilter::Create(region, 0.2f, 0.7f))->unref();
+    paint.setImageFilter(SkAlphaThresholdFilter::Make(region, 0.2f, 0.7f, nullptr));
     return paint;
 }