Proposal: offerDataChannel in RTCOfferOptions

Currently, RTCPeerConnection has "offerToReceiveVideo" in the
RTCOfferOptions, which is a convenience to allow creating an offer with a
m-line for video without first having to attach a video MediaStreamTrack.
This is useful; without it, pages would have to attach a track and then
renegotiate, even if they only want to receive video.

However, with data channels, we don't have this convenience.  The offerer
must call createDataChannel before createOffer, or suffer a renegotiation,
even if they have no immediate need for a data channel.

So what do you think of adding a boolean "offerDataChannel" attribute to
RTCOfferOptions, parallel to "offerToReceive*"?  I think that would be
easier to use.

--Ben