blogged about this development providing a couple more technical details. As one might expect Mir development being focused mainly on mobile devices so far (phones, tablets) it doesn't provide Window Management.
I am calling it ?proof of concept? not because of the quality of the code, but because there is more work to do to fully integrate Chromium with Mir, e.g. Mir does not provide Window Management, so Chromium will excel to its fullest extend only on Unity 8 on Mir.