Have OverviewButtonTray observe WindowSelectorController changes
OverviewButtonTray changes its active state in response to user touch and activation. It would dismiss the active state when pressed again. However Overview Mode can be dismissed by other actions.
Implment and Observer in WindowSelectorController to notify of entering/exiting Overview Mode. Have OverviewButtonTray listen and update its state on dismissal.
TEST=OverviewButtonTrayTest.ActiveStateOnlyDuringOverviewMode
BUG=419875
Review URL: https://codereview.chromium.org/
671303005
Cr-Commit-Position: refs/heads/master@{#301660}