GitLab

Add a check if the user tries to create a profile with the
name of an already existing supervised user. In that case,
the user is not allowed to create a new such supervised user
profile, instead he can directly import the existing supervised
user.
A screenshot can be found in the bug description.

BUG=282464
TEST=browser_tests

Review URL: https://codereview.chromium.org/125993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243855 0039d316-1c4b-4281-b951-d872f2087c98