cros: Add a multiprofile first run notification.

Show a notification when user signs into a multiple profile session
for the first time.

BUG=293641
R=dpolukhin@chromium.org, mnissler@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/24227011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225090 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225090