Brave Browser has some Rust components too (e.g. the adblocking engine). Rust in C++ works very well for components with a small, well-defined API surface that involve a lot of logic. You only need to build a small FFI layer for the direct interfaces between the two languages. CXX [1] makes this even easier, too.

Brave is definitely not aiming to convert the entire browser to Rust, but it's increasingly chosen for new development.

[1] https://cxx.rs/