Virtual Desks: Add a chrome flag to enable the feature

The flag will be disabled by default in M-76.

BUG=866622
TEST=Manually, AboutFlags*Test*

Change-Id: I9a10ad2b933625e7c7e997d1f7e7c3bededc9396
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627653
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664990}
5 files changed