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One feature of the mocks that is yet to be implemented is the handling
of backup-location choice/re-use. In this CL we implement those
features. Specifically:
 - Backup will use the default location without bringing up a
   file-chooser (unless it is taken, in which case the file chooser is
   brought up to confirm the overwrite/rename)
 - If the user selects "change location" the file chooser will be shown.
 - If backup completed successfully and upgrade fails, restore will
   immediately begin from the known backup location, rather than
   prompting the user to find their backup.

Bug: 1024693
Change-Id: Iaa57f7162e7c12b483ba30b205c720dd88ea0c31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068400
Commit-Queue: Nic Hollingum <hollingum@google.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745370}