Bug 1600019 - Part 3: Use NewObjectWithGivenProto when |prototype| argument is guaranteed to be non-null. r=jonco

`NewObjectWithClassProto` when called with a non-null prototype calls `NewObjectWithGivenTaggedProto`,
but that function can be called more directly through `NewObjectWithGivenProto`.

Depends on D55087

Differential Revision: https://phabricator.services.mozilla.com/D55088

--HG--
extra : moz-landing-system : lando

js/src/builtin/FinalizationGroupObject.cpp

@@ -665,7 +665,7 @@ bool GlobalObject::initFinalizationIteratorProto(JSContext* cx,
   }
 
   FinalizationIteratorObject* iterator =
-      NewObjectWithClassProto<FinalizationIteratorObject>(cx, proto);
+      NewObjectWithGivenProto<FinalizationIteratorObject>(cx, proto);
   if (!iterator) {
     return nullptr;
   }