WebRTC part of screen sharing experiment added

You can share part of webpage/screen; part of screen can be a <div>, an
<aside> or any HTML element.

Try part of screen sharing on Firefox here:
https://googledrive.com/host/0B6GWd_dUUTT8RzVSRVU2MlIxcm8/realtime-chat/

Now, using random data ports for RTCDataChannel (SCTP streams) on
Firefox for multi-user file/text/data sharing.

https://webrtc-experiment.appspot.com/chat-hangout/
https://webrtc-experiment.appspot.com/file-hangout/

Added support for android (chrome beta), but haven't tested yet.