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This CL makes sure the control buttons view in arc notifications
is not set to visible when the message center is collapsed.
Also fixes GetLastFocusableChild to account for arc notifications.

Bug: 1057988
Change-Id: If4e9988e7362783f4e0331b7bf8cb3c2fe2ce447
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114166Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752648}