Add boolean preference PluginVmImageExists

Add boolean preference PluginVmImageExists representing whether there is
a PluginVm image for a user on a device. This preference shouldn't be synced.

Bug: 904848
Change-Id: Ie90b673fe33fa66533f475c667bc3611e1030b88
Reviewed-on: https://chromium-review.googlesource.com/c/1414855
Commit-Queue: Olya Kalitova <okalitova@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623388}
6 files changed