[webrtc-stats] RTCRemoteInboundRtpStreamStats: total round trip time (#426)

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

== RTCRemoteInboundRtpStreamStats: total round trip time ==
[RTCRemoteInboundRtpStreamStats.roundTripTime](https://w3c.github.io/webrtc-stats/#dom-rtcremoteinboundrtpstreamstats-roundtriptime) is only the [latest estimate](https://tools.ietf.org/html/rfc3550#section-6.4.1) (scroll down to "delay since last SR (DLSR): 32 bits").

To allow calculating averages we should do the same thing that we did with regards to RTCIceCandidatePairStats - have a `totalRoundTripTime` and a counter that gets up every time you calculate a sample, i.e. every time you get a Receiver Report... or more accurately, every time you get a Report Block; `reportsReceived`.

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/426 using your GitHub account