Given the same base OS (Linux) in Android and ChromeOS and much of the same sorts of things. The redundancy of having two teams is pretty obvious. So if everyone agrees that you're going to do both going forward (and I think this is a ringing endorsement of ChromeOS as a 'long term' bet by Google) it only makes sense to make one team out of them.
Then the question becomes who leads it? The guy who leads ChromeOS or the guy who leads Android? Andy wasn't a big fan of ChromeOS when I was there, I don't know if that changed, his passion was making a better phone experience. I don't think I ever talked to or hear Sundar speak when I was there so I don't know what he thinks about phones.
So Sundar gets the nod and Andy gets to pick his next role (or exit which isn't uncommon in these situations). I don't doubt Andy would be on the short list of a number of CEO search committees so perhaps he'll pop up as the new CEO of Sony or something.