GitLab

- Prevent Google Credential Provider for Windows from starting another
instances of Chrome on winlogon screen if one is already running
- Fix DCHECK failure that would crash the GCPW if the user cancels out
of the sign in or selects a different credential to sign into.

Bug: 900966
Change-Id: Ib30b4eff282cb55fa3fe6777dca56ff2051e275c
Reviewed-on: https://chromium-review.googlesource.com/c/1334293
Commit-Queue: Tien Mai <tienmai@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608804}