WebRTC.MediaHandler: don't pass constraints to peerConnection.addStream

This argument:

* [was removed from the current draft WebRTC spec](http://dev.w3.org/2011/webrtc/editor/archives/20140617/webrtc.html#changes-since-august-30-2013) (no. 16)
* [was optional before it was removed](http://dev.w3.org/2011/webrtc/editor/archives/20130830/webrtc.html#widl-RTCPeerConnection-addStream-void-MediaStream-stream-MediaConstraints-constraints)
* doesn't seem useful to applications

Also, this facilitates compatibility with WebRTC plugins (such as [Temasys's](https://temasys.atlassian.net/wiki/display/TWPP/WebRTC+Plugins))
that cause an error if the argument is passed.