Remove dependency on the BrowserListObserver from ToastController

The toast controller needs to know when the browser is starting to close
so that it can clear out all queued toasts before an existing toast
closes so that we don't try to show those queued toasts. This CL
adds a way to register with the BrowserWindowInterface to be notified
when the active tab changes and checks whether the browser is attempting
to close.

Bug: None
Change-Id: I3209be8ff7d279764f7a00942e5f075040e005da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5866649
Reviewed-by: Erik Chen <erikchen@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Steven Luong <stluong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359505}