Multiple project want some notion of "user initiated" tab/window closure.

e.g:
1. We want to collect metrics on spammy popups that users close right away
2. We want to use this for page load abort tracking (did the user close the page while it was loading?

Right now, this signal can be corrupted by script-initiated close (e.g. window.close()) or other non-user initiated close (extensions, background contents for offline, etc).

I *think* the best signal for this is logic flowing from browser_commands.cc (CloseWindow/CloseTab), but I'm not confident about Android.