Disable force-signin on Linux

Force-signin is not launched on Linux. When BrowserSignin is set to Force(2),
change it to Allow(1) instead.

Bug: 956998
Change-Id: I33a6fe51d4a3707fff4fa8cc7430509e262d3270
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1686778
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674837}