iLBC codec not offered in WebRTC media sessions.

Many PTSN gateways still only have iLBC as their only low-bandwidth gateway. WebRTC supports iLBC but Chrome has it switched off in an attempt to force everyone to move to Opus. While this is admirable its not practical.

Switching iLBC on is very simple and has no impact on Opus being the preferred codec. I've made the changes and we have tested the codec. Pull request will follow. 