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We extend the BorealisWindowManager to track the lifetime of individual
windows, apps (as collections of windows) and the session (as
collections of apps.

This new observer was kept separate from the already-present anonymous
observer as the two have limited overlap, and are not used
simultaneously by any clients (currently).

We also implement a trivial watcher in the BorealisContext which joins
the new session-observing behaviour with the ShutdownMonitor. This
watcher will only be active while the context exists so we will not
accidentally issue shutdown requests while the vm is not running.

Bug: b/172006764, b/172979315
Change-Id: Ie58c87acbb14eebe41152986f6b86032a5d8c759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576876
Commit-Queue: Nic Hollingum <hollingum@google.com>
Reviewed-by: Daniel Ng <danielng@google.com>
Cr-Commit-Position: refs/heads/master@{#835006}