The main question is what behavior is being introduced. I haven't researched deeply, but apparently the add-on does nothing until the user opts-in on studies.
An appropriate response here would be to decide that you no longer trust their browser at all.
It's hard to quantify trust exactly. I'm fine with trusting the partly-closed-source Google Chrome build, including the proprietary Chromecast, Hangouts, etc., plugins, because I believe that the people writing them are generally reasonable. I don't have a good formal proof that they're generally reasonable people, and I never will - that's why it's trust. If they start installing marketing gimmicks, certainly they have the technical ability to do that, but I will lose my trust that they're reasonable people.
Here's an analogy: I trust a small number of my friends with keys to my apartment because I think they'll make reasonable use of that access. If they decide to show up at 3 AM with a keg and three tubas without telling (let alone asking) in advance, I technically have no grounds to complain that they abused their access - but I'll certainly not be calling them friends any more.
Hence, as you said, the only way is to trust Google here, without much ability to verify.