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/include/effects/SkLayerDrawLooper.h b/include/effects/SkLayerDrawLooper.h
index 6ddde88..186d44a 100644
--- a/include/effects/SkLayerDrawLooper.h
+++ b/include/effects/SkLayerDrawLooper.h
@@ -80,7 +80,7 @@
     SK_TO_STRING_OVERRIDE()
 
     Factory getFactory() const override { return CreateProc; }
-    static SkFlattenable* CreateProc(SkReadBuffer& buffer);
+    static sk_sp<SkFlattenable> CreateProc(SkReadBuffer& buffer);
 
 protected:
     SkLayerDrawLooper();