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Introduces a UILockController owned by Seat which listens for keypress
events, and minimizes the focused non-immersive fullscreen window. The
intention is for UILockController to also release other kinds of "lock"
(such as pointer lock) in future, probably via a publish-subscribe
mechanism exposed by Seat.

Caveats:

* UX is not final. Minimizing may not be the right action; using it for
  now since switching to windowed mode currently causes incorrect
  rendering for some games.
* We'll need UI to communicate this affordance to users.

Bug: b/161952658
Change-Id: Ibbfa28d473e5b152dc59dcb5fda0c2c99496dd0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315718
Commit-Queue: Chloe Pelling <cpelling@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Reviewed-by: Daniel Ng <danielng@google.com>
Cr-Commit-Position: refs/heads/master@{#802136}