Make chat more usable during audio only calls.

When sending an audio+video offer, if the answer contains audio only, then the data channel isn't reliably established for some reason, so I would recommend that someone demoing chat ensures that the call is started as audio-only.

Otherwise things seem to be in good shape for a chat demo.
I didn't find a clean CSS way to make the chat area take all the available vertical space on the mobile page, so I have hardcoded height values that fit well for the Nexus 4 (there's a media query, with one value for landscape and one for portrait mode; both modes should be usable for a chat demo).