Do not show first-run UI in public session.

BUG=344869
TEST=manually

Review URL: https://codereview.chromium.org/166323008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254275 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254275