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This is the first step to allow users to install Crostini with a custom
username.

Bug: 1016195
Change-Id: I18b6bd3c99198ff733c987dd25ae2d46029a6c9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880411Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#712047}