Handle completeAuthentication for lockscreen online re-authentication

complete the user authentication after they are authenticated by IdP
page and re-open the session.

Bug: 1102942
Change-Id: I749f484462758a9709ee7f6a1db225b423d2a1dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440529


Commit-Queue: Mohammed Abdon <mohammedabdon@chromium.org>
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850531}