And since git is distributed it can be hosted on both, and already is https://github.com/v8/v8-git-mirror
Aren't mirrors going to be slightly out of date (unless continually synchronised). And you wouldn't commit to the mirror, right?
For me the advantage is being able to use GitHub's nice UI to browse the source code.
Synchronising a mirror automatically is pretty trivial so I imagine they're usually pretty up to date.
fwiw you can do this on chromium.googlesource.com as well. look for link called `[path history]` when navigating the tree.