Set focus to the "OK" button when editing a supervised user profile

Since a supervised user can't edit their name, trying to set the focus there
resulted in it ending up on the first element in the tab order. On Mac and
Linux, that was the "Cancel" button, rather than "OK", because the button order
is reversed to fit those platforms' conventions. Set focus more deliberately.

BUG=315532
TEST=new browser_test

Review URL: https://codereview.chromium.org/75363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235754 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235754