What do we know about marketing gimmicks hidden in Chrome? If they are not made in the form of add-ons, or if they are add-ons but Chrome has a way to hide them (as it hides Flash), we just never know. I bet Google's marketing gimmicks, if any, are not open-source either, and not included in Chromium.

Hence, as you said, the only way is to trust Google here, without much ability to verify.