GitLab

As a tabmodel needs to be deprecated, this CL extracts
ClosingWebStateObserver into a browser agent to remove its usage in
TabModel.

Bug: 1050111,1045575,1045142
Change-Id: Ic6c5b65ff55bd211d38752b274c4dcbb9d854bcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2287434
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800977}