Add UI for supervised users re-creation (under flag).
Also sets correct avatar id on ChromeOS.

BUG=282464

Review URL: https://codereview.chromium.org/23710068

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229018 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229018