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This CL imposes restrictions on the username in crostini installer to
ensure the username conforms with linux convention and does not conflict
with other default users. If the username is invalid, an error message
will be shown and the install button will be disabled.

The CL also adds new browser tests for this feature. Since the flag
crostini-username is turned on for all installer browser tests now,
existing browser tests are modified to click the "next" button to go
into the configuration page before clicking the "install" button.

Bug: 1016195
Test: browsertest; manual test with flag crostini-username flag on
Change-Id: I546e0c56a3a41defb2de3e224992cebf5824dabc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032485
Commit-Queue: Jason Lin <lxj@google.com>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737686}