Disable broken UserAddingScreenTest

This test tries to verify Chrome OS multi-profile behavior but does not actually
enable multi-profile support.

BUG=326832
TBR=dzhioev@chromium.org

Review URL: https://codereview.chromium.org/109703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239395 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239395