Proposal: Replace offerToReceiveVideo with PeerConnection.createRtpReceiver.

It looks like we'll soon be adding PeerConnection.createRtpSender, which is
a way to create an RtpSender before one has a track to send.

The logic corollary is PeerConnection.createRtpReceiver, which would be a
way to create an RtpReceiver before any media negotiation has occurred.  In
other words, it would serve the same purpose as "offerToReceiveVideo".

In both implementing offerToReceiveVideo and using it in JS, I found it
awkward.  I think the following example code would be more natural:

var receiver = pc.createRtpReceiver("video");
// The offer now acts like it currently does with "offerToReceiveVideo = 1".
var offer = pc.createOffer();
// JS can use the receiver.track immediately.


I think this would be more natural, and more logically consistent with
PeerConnection.createRtpSender.


If you'd like to see how it would look in a PR, here it is:

https://github.com/w3c/webrtc-pc/pull/279/files