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This will complete the flow when calling SignIn() and will allow the UI to be
notified if the user signs in or out of the browser.

Bug: b/170751292
Change-Id: I64a7a45b50d545596cfafe3b3335a1de8d94f71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551134
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832209}