[iOS] Notify BrowserObservers of TestBrowser destruction.

This allows BrowserObservers to remove themselves in tests to avoid
the checks_empty=true DCHECK in the ObserverList.

Bug: none
Change-Id: I3d87782a29c72b800333cd9eb4b79244daf45adb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598874
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657370}
1 file changed