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This re-lands commit a5646d5a.

Difference to original:

Override TearDownOnMainThread() instead of TearDown().

Original change's description:
> Remove NOTIFICATION_FULLSCREEN_CHANGED
>
> Replace with an observer interface on FullscreenController.
>
> Bug: 268984
> Change-Id: I1af2056e870af56b2f436a3aa8c6a54ed0ff44fb
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666148
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Evan Stade <estade@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#671901}

Bug: 268984
Change-Id: I735754efa634d8ef21b4bb57f366dbad22b3c682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676731
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673412}