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This allows lacros/chrome to auto hide/show ChromeOS shelf
when it enters in fullscreen mode. It is the so called
'immersive fullscreen' mode that chromeos/chrome has.

MANUAL TEST: Enter lacros/chrome in fullscreen and hover
the mouse pointer over the bottom border of the screen
(shelf area). The shelf should slide-up.

Note that this brings lacros/chrome fullscreen behavior
closer to chromeos/chrome's. In a follow up CL, the omnibox
widget that slides down from the upper border of Ash will
be plumbed up to lacros/chrome.

BUG=1113900
R=msisov@igalia.com, nickdiego@igalia.com

Change-Id: I4b7c04d18b73747a0c70bffa5a6d6741df6c4346
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353116Reviewed-by: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Commit-Queue: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#798117}