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/tools/debugger/SkOverdrawMode.cpp b/tools/debugger/SkOverdrawMode.cpp
index 7eee387..1e018ba 100644
--- a/tools/debugger/SkOverdrawMode.cpp
+++ b/tools/debugger/SkOverdrawMode.cpp
@@ -313,8 +313,8 @@
     typedef SkXfermode INHERITED;
 };
 
-SkFlattenable* SkOverdrawXfermode::CreateProc(SkReadBuffer& buffer) {
-    return Create();
+sk_sp<SkFlattenable> SkOverdrawXfermode::CreateProc(SkReadBuffer& buffer) {
+    return sk_sp<SkFlattenable>(Create());
 }
 
 sk_sp<SkXfermode> SkOverdrawMode::Make() { return sk_make_sp<SkOverdrawXfermode>(); }