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/PDFPrimitivesTest.cpp b/tests/PDFPrimitivesTest.cpp
index 57b0b60..f4c29b3 100644
--- a/tests/PDFPrimitivesTest.cpp
+++ b/tests/PDFPrimitivesTest.cpp
@@ -381,10 +381,10 @@
     mutable bool fVisited;
 };
 
-SkFlattenable* DummyImageFilter::CreateProc(SkReadBuffer& buffer) {
+sk_sp<SkFlattenable> DummyImageFilter::CreateProc(SkReadBuffer& buffer) {
     SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 0);
     bool visited = buffer.readBool();
-    return new DummyImageFilter(visited);
+    return sk_make_sp<DummyImageFilter>(visited);
 }
 
 #ifndef SK_IGNORE_TO_STRING