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This allows custom launcher pages to be torn down in response to this
notification in the same way that regular packaged app windows are.
Currently these WebContents are torn down later, by
Widget::CloseAllSecondaryWidgets(), but this relies on the WebContents
lifetime being tied to a Widget.

Observing NOTIFICATION_APP_TERMINATING also allows us to explore
decoupling the AppListViewDelegate from the AppListView lifetime, and
remove some special code on Mac to deal with updating the profile
switcher (since the AppListViewDelegate is owned by a leaky singleton
there).

BUG=403647

Review URL: https://codereview.chromium.org/559823002

Cr-Commit-Position: refs/heads/master@{#295194}