[webrtc-pc] Deprecate RTCSessionDescription constructor or throw error on missing sdp

jan-ivar has just created a new issue for 
https://github.com/w3c/webrtc-pc:

== Deprecate RTCSessionDescription constructor or throw error on 
missing sdp ==
The spec says of 
[sdp](http://w3c.github.io/webrtc-pc/#dom-rtcsessiondescriptioninit-sdp):
 *"if type is rollback, this member can be left undefined."* but no 
processing steps to say what happens if it's undefined for other 
types. E.g. What does this output?

    console.log(new RTCSessionDescription({type: "answer"}).sdp);

Alternatively, we could deprecate the constructor since it is no 
longer needed for anything.

Omitting sdp in the set description processing steps already yields 
`InvalidModificationError`, so I believe we are fine there.

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/897 using your GitHub account