Multi Profile: Adding hotkey to switch between logged in users

BUG=296271
TEST=visual

Review URL: https://codereview.chromium.org/23533078

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225396 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225396