I don't want to discourage the developers working on this project, but I'm curious why we're still writing applications that will almost certainly execute or process hostile content in languages that don't maintain strict memory safe contract? Have we not learned our lesson yet, or am I misunderstanding the situation? I believe it was a Microsoft study that linked unsafe memory access to ~70% of exploit chains.
That point is 50% FUD. A language which "maintains strict memory safe contracts" but has an `unsafe` keyword; or has a "native code interface", or uses libraries implemented in a different language, doesn't really strictly maintain its guarantees. And on the other hand, a language in which you can, in principle, load and execute arbitrary code from a string you got from the user, can be hardened very well by statically-checkable constraints.
So, it's a matter of degrees rather than absolutes. If you then add considerations such as programming paradigm flexibility and performance, C++ is very much a valid choice even for the use case of a browser.