[ios] [BVC refactor] Prevent crash in BrowserCoordinator |-stop|

A coordinator's |-stop| should not crash if called multiple times.

Bug: 910652
Change-Id: I5a2998726ad3390a8b1c98afbc753f093a4a8986
Reviewed-on: https://chromium-review.googlesource.com/c/1357001
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612706}
1 file changed