Have Demo account logins follow their own login path.
Currently if a demo account signs in, we incorrectly follow the path for a regular user login, which does additional work that we do not want for demo accounts (creates user pods, set the oauth status). Instead we now will use our own demo user logged in path which only sets up the user aspects that we want set up.

R=xiyuan@chromium.org
BUG=346482
TEST=A demo session starting does not create a user pod.

Review URL: https://codereview.chromium.org/181613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253772 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253772