The spec adds `captureTimestamp` and `senderCaptureTimeoffset` to [`RTCRtpContributingSource`](https://w3c.github.io/webrtc-extensions/#rtcrtpcontributingsource-extensions), yet the [corresponding test](https://github.com/web-platform-tests/wpt/blob/master/webrtc-extensions/RTCRtpSynchronizationSource-captureTimestamp.html) (and apparently the [implementation in Chromium](https://wpt.fyi/results/webrtc-extensions/RTCRtpSynchronizationSource-captureTimestamp.html?label=experimental&label=master&aligned)) has them in `RTCRtpSynchronizationSource`.