Chromium took parts from WebKit, but e.g. the whole JavaScript engine was completely new. Even if they needed to depend on some libraries written in C++, that doesn't justify writing the whole application in that language. And once they forked WebCore as Blink they could have gradually pushed the borderline down.
I wish I had read this comment before I replied to your parent, if I had thought about a primary Haskell or OCaml implementation with FFI and then gradually shifting I might have just kept my mouth/keyboard shut.
