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NotificationControlButtonsView is added to a NotificationHeaderView
which is then added to the NotificationViewMD.

This means NotificationViewMD maintains the lifetime of
NotificationControlButtonsView as long as it doesn't remove the
NotificationControlButtonsView as a child.

BUG=1044687

Change-Id: Ic1b24832f7a26573a621604a56f5441c982b685a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202534
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Auto-Submit: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770061}