Implement async fullscreen operation behind a flag.

This CL adds a way to asynchronously set window state fullscreen.
The behavior is currently tested only on Lacros to start from
smaller scope.
Adding the feature flag also helps us to check the behavior
in other platforms, and will be tested more.

BUG=40276379
TEST=Tryjob

Change-Id: Ifdbc79daac7022d32ac1a0c43ea7e8c657b9078b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5633160
Reviewed-by: Mike Wasserman <msw@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326783}