[css-view-transitions-2] ViewTransition.types should be [SameObject] (#13140)

As it is expected to return the same object all the time.

Author: emilio

css-view-transitions-2/Overview.bs

@@ -1719,7 +1719,7 @@ Issue: Is this updated by a <code>rootEl.startViewTransition()</code> call, too?
 			readonly attribute Promise<undefined> ready;
 			readonly attribute Promise<undefined> finished;
 			undefined skipTransition();
-			attribute ViewTransitionTypeSet types;
+			[SameObject] attribute ViewTransitionTypeSet types;
 			readonly attribute Element transitionRoot;
 			undefined waitUntil(Promise<any> promise);
 		};