rAc: prevent users from clicking on item separators.

R=groby@chromium.org
BUG=347306

Review URL: https://codereview.chromium.org/182503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253979 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253979