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This is needed because at the current time we have no way to get such a
list, and we arguably shouldn't be trying to handle containers we didn't
set up anyway. The pref is a list of dictionaries, with values for
"vm_name" and "container_name". By default, the pref will be set to
[{vm_name: termina, container_name: penguin}] to handle existing users.

Change-Id: I69437fc3e7675da9058d9f83436edc82c5dc91db
Reviewed-on: https://chromium-review.googlesource.com/c/1429320
Commit-Queue: Fergus Dall <sidereal@google.com>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626994}