Add test for VC sharing fix.

When the active user is switched, the screen access should be stopped.
However, if the Video Conference feature was enabled,
ScreenSecurityController::OnScreenAccessStop returned early without
calling StopAllSessions, leaving the screen share active.

Bug: 502684181
Test: covered by test
Change-Id: I5f7a54e95619662cd60384089bffa1189d739f96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7855461
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1632433}