[rasvj]

Both Firefox's and Chromium's source code is completely open (it could be argued that Chromium's code is even more open since it's mostly BSD). Google can't stop you from modifying anything in the browser, and that includes the address bar. The language in which that component is implemented is irrelevant, and Google has certainly not chosen C++ to implement to address bar with the express purpose of making that component harder to modify.

[zuzun]

If there's no real API, you have to patch things and diverge from master and then you face the problem that Chromium is under constant change that you most certainly can't keep up with, so either you chose to ignore all the work happening in Chromium and end up with a stale branch or you spend most of your time making your patch work with the latest changes.