cros: Make links on enrollment error screen accessible via keyboard.

Add href attributes to those links so that tab stops on them.

BUG=304988

Review URL: https://codereview.chromium.org/26390002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227487