This is already happening for a long time. I remember some ad banners which looked like message boxes or download dialogs in Windows XP style.
(... which were easy to spot for me, because I'm using a completely different system)
(... which were easy to spot for me, because I'm using a completely different system)
It's a pretty cool feature, but it means that on Firefox, attackers should be able to emulate basically any chrome they want to.