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Previously if the user cancelled the backup, we would proceed with the
upgrade as though the user had elected not to backup at all.

With this change, the flow will instead return to the initial screen
when the user cancels, rather than proceed.

Bug: 1061203
Change-Id: I7be99a2c7a6caa4b5fd143def5ba0c6f9b4d5004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102935Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#750833}