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The fullscreen toolbar controller currently uses undocumented Carbon
events to track the menubar's reveal amount in fullscreen. In recent
macOS versions, the intermediate values are no longer send and we
only receive events when the menubar is fully shown or fully hidden.

The Mac immersive code which is still behind a flag uses a different
technique, observing the reveal amount on a hidden titlebar accessory
view. This *does* provide correct intermediate values.

This change extracts the reveal code from the immersive controller
and uses it in the current fullscreen code rather than the Carbon
event. Unfortunately, despite receiving the correct progress
notifications, the topchrome still does a discontinuous jump, but
we'll cross that bridge later.

Bug: 1063417
Change-Id: I6827cafbcad58eeaabf98c6facaeb85ce3439997
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314998Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791273}