Stop the OverviewButton from appearing on the Add-User screen
OverviewButtonTray was showing up on the Add-User screen. It should only be
available when a user is logged in and in the active state.
Update OverviewButtonTray to observe SessionState changes to determine
visibility.
Update WindowSelectorController to not allow overview mode while on the
add-user screen.
TEST=OverviewButtonTrayTest.VisibilityChangesForLoginStatus
BUG=488464, 482591
Review URL: https://codereview.chromium.org/1149833006
Cr-Commit-Position: refs/heads/master@{#332872}
3 files changed
