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When shutting down overview session, make sure the
OverviewModeFocusedWidget is hidden (and deactivated), to prevent it
from handling key pressed events (when the overview session is active,
key press events are handled by OverviewSession before they get
dispatched to the active window).
Without this, the OverviewModeFocusedWidget might end up handling tab
key event, and cause a crash when the focus manager attempts to find the
next focusable view within an childless widget.

BUG=1033972

Change-Id: I0b817cb5784b6ffd79d5a4b011a233cd410863c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036258Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738031}