I'm not sure that a CDN comes into play when doing video chat. A CDN is for replicating content and allowing your number of viewers to scale up. With VideoChat you are not really replicating content. If you need to add multiple servers to scale up your video chat service, you would need to do some custom programming to ensure users involved in a single chat are all directed to the same server.
All other things being equal, using a CDN will add the latency that exists between you and the CDN. Whereas if all your edge servers were on the same LAN as your origin, you would not have that extra latency.