Right now the Chromium codebase have less bugs than if it was written in Rust, because of how many hours and efforts that were spent on the codebase to make it work smoothly.
Also the particular engineers with particular know-hows work with a particular tool. Its not easy to find them especially if you are forcing to use a new language in a giving project. It will take years and even decades to have the same quality people you already have in C and/or C++ in Rust.
So it will take time to have an engineer that can make, lets say, a multi-process compositor in Rust. What about many people with that particular special knowledge that take years to be good at.
So in real life its not just a matter of a language being a better fit. Theres a lot of other vectors to take into account.
https://www.joelonsoftware.com/2000/04/06/things-you-should-...
To elaborate on the other commenter, the majority of bugs found in chromium are bugs that rust would avoid (memory safety issues that rust makes ~impossible).