[webrtc-stats] Stat for likelihood of echo

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

== Stat for likelihood of echo ==
The non-standardized Chromium getStats contains the following stat:
```
// An instantaneous measurement of the likelihood that there is echo.
// This is a send side stat with range 0 (no echo) to 1 (definitely 
echo).
// A value of 0.5 (may need updating) indicates very likely echo.
ssrc.googResidualEchoLikelihood
```
Should we standardize this? `double residualEchoLikelihood` in 
RTCMediaStreamTrackStats?

Please view or discuss this issue at 
https://github.com/w3c/webrtc-stats/issues/153 using your GitHub 
account