[webrtc-stats] new commits pushed by vr000m

The following commits were just pushed by vr000m to https://github.com/w3c/webrtc-stats:

* Clarify Cumulative metric for XR blocks and not interval (#490)

If these are reported in XR blocks, these metrics should be in RTCReceivedRtpStreamStats and not in RTCInboundRtpStreamStats
  by Varun Singh
https://github.com/w3c/webrtc-stats/commit/755c1f9e6a4c24ad4a6097ff1eeeccdc3457da15