change flattenable factory to return sk_sp

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1855733002

Review URL: https://codereview.chromium.org/1855733002
diff --git a/tests/ImageFilterTest.cpp b/tests/ImageFilterTest.cpp
index dda899b..3f60286 100644
--- a/tests/ImageFilterTest.cpp
+++ b/tests/ImageFilterTest.cpp
@@ -219,12 +219,12 @@
 
 }
 
-SkFlattenable* MatrixTestImageFilter::CreateProc(SkReadBuffer& buffer) {
+sk_sp<SkFlattenable> MatrixTestImageFilter::CreateProc(SkReadBuffer& buffer) {
     SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 1);
     skiatest::Reporter* reporter = (skiatest::Reporter*)buffer.readFunctionPtr();
     SkMatrix matrix;
     buffer.readMatrix(&matrix);
-    return new MatrixTestImageFilter(reporter, matrix);
+    return sk_make_sp<MatrixTestImageFilter>(reporter, matrix);
 }
 
 #ifndef SK_IGNORE_TO_STRING