Adding spacing between (multiple) user items in system tray

BUG=305064
TEST=visual

Review URL: https://codereview.chromium.org/27226005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228667 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228667