[webrtc-stats] Make sure timing and responsibility for stat object creation is clear

stefhak has just created a new issue for 
https://github.com/w3c/webrtc-stats:

== Make sure timing and responsibility for stat object creation is 
clear ==
>From web perf group review [1]
"t does not appear that this specification clearly defines when the 
various Stat objects are created and what is responsible for emitting 
the various Stat objects. Is another specification responsible for 
that?

It would be useful to link to that as it seems critical to ensuring 
cross-browser compat."

[1] 
https://lists.w3.org/Archives/Public/public-web-perf/2016Dec/0014.html

Please view or discuss this issue at 
https://github.com/w3c/webrtc-stats/issues/120 using your GitHub 
account