That post describes two vulnerabilities: one is in the JIT, but the other one is in regular old C++ code. More generally, JIT bugs are a relatively small minority of browser vulnerabilities. More often you see issues like use-after-free in C++ code that interacts with JS, such as implementations of DOM interfaces, but the issues are not directly JIT related and would be avoided in a fully memory-safe language.
Chrome, like Firefox, is not an example of modern C++ code. Google's and Mozilla's coding standards enforce a late-'90s style. It is astonishing they get it to work at all.
