While I'm generally in favor of the proposition that C++ is an intrinsically dangerous language, pointing at one of the largest possible projects that uses it isn't the best argument. If I pushed a button and magically for free Chrome was suddenly in 100% pure immaculate Rust, I'm sure it would still have many issues and problems that few other projects would have, just due to its sheer scale. I would still consider it an open question/problem as to whether Rust can scale up to that size and still be something that humans can modify. I could make a solid case that the difficulty of working in Rust would very accurately reflect a true and essential difficulty of working at that scale in general, but it could still be a problem.
(Also Rust defenders please note I'm not saying Rust can't work at that scale. I'm just saying, it's a very big scale and I think it's an open problem. My personal opinion and gut say yes, it shouldn't be any worse than it has to be because of the sheer size (that is, the essential complexity is pretty significant no matter what you do), but I don't know that.)
You're right that Chromium* is a very difficult task, but I disagree with the conclusion you draw. I think Chromium is one of the best examples we can consider.
There would absolutely be issues, including security issues. But there is also very good evidence that the issues that are most exploited in browsers and operating systems relate to memory safety. Alex Gaynor's piece that the author linked is good on this point.
While securing Chromium is huge and a difficult task, it and consumer operating systems are crucial for individual security. Until browsers and consumer operating systems are secure, individuals ranging from persecuted political dissidents to Jeff Bezos won't be secure.
* Actually not sure why I said Chromium rather than Chrome. Nothing hangs on the distinction, afaict.
(Also Rust defenders please note I'm not saying Rust can't work at that scale. I'm just saying, it's a very big scale and I think it's an open problem. My personal opinion and gut say yes, it shouldn't be any worse than it has to be because of the sheer size (that is, the essential complexity is pretty significant no matter what you do), but I don't know that.)