Modernize WebRTC with promises compatibly with hybrid.

Let me know if I missed anything. Leaving polish of legacy note to others.
I remember that the first sentence has been there for a long time. Is it perhaps the third sentence that has been added later? I think we have quite a bit of redundancy for two sentences is the same paragraph.

Sentence 1:
Session descriptions generated by createOffer MUST be immediately usable by setLocalDescription without causing an error as long as setLocalDescription is called within the successCallback function.

Sentence 3:
The session descriptions MUST remain usable by setLocalDescription without causing an error until at least end of the successCallback function.