[GCM] Add ConnectionListener support, and hook up to debug page
The ConnectionFactory::ConnectionListener gets informed of connection events,
including the actual ip endpoint with which a connection was established. This
will eventually be hooked up to app handlers to allow services to act on
connection changes. For now it just allows the debug page to more accurately
reflect the connection state.
BUG=377882
Review URL: https://codereview.chromium.org/
317723004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276800 0039d316-1c4b-4281-b951-d872f2087c98