cros: Fix NOTREACHED() in login screen for multiprofile.

Follow-up CL of r224862 to fix the NOTREACHED() in
ProfileHelper::GetUserIdHashFromProfile on login screen when multiprofile is
enabled.

BUG=140591
TEST=No NOTREACHED() when running chrome as login manager with multiprofile enabled.
R=dpolukhin@chromium.org

Review URL: https://codereview.chromium.org/23702059

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225101 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225101