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This reverts commit a5646d5a.

Reason for revert:

Crashes 5 tests on Win 7 Tests x64 (1):
https://ci.chromium.org/p/chromium/builders/ci/Win%207%20Tests%20x64%20%281%29/55573

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}

TBR=sky@chromium.org,estade@chromium.org

Change-Id: I793c04325fa1e4e9e6fb07256e01ff81c22e3b0d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 268984
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672139Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671920}