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- Add a feature flag for desks-restore.
- Store a primary user active desk in user prefs and restore it
after a crash and a logging out.

# TODOs
- crbug.com/1143997 : secondary users should restore to a
default active desk when the crash happened in their sessions.
- crbug.com/1145404 : consider adding an UMA metric for restoring
desks.

Bug: 996999
Test: Manual test, ash_unittests --gtest_filter=DesksRestoreMultiUserTest.DesksRestoredFromPrimaryUserPrefsOnly
Change-Id: I301b394b147468474233af507a263a2d9d38fe53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507399Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Cattalyya Nuengsigkapian <cattalyya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826895}