In the new default multi profile case, all user icons have to be added to the system tray. Made them also clickable to allow to switch to new users.

The final UI might change a bit, but this is basically it.

BUG=305064
TEST=ash unit tests, and visual

Review URL: https://codereview.chromium.org/26164009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228076 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228076