Define the reference clock used for RTCRtpContributingSource.timestamp (#1854)
* Define the reference clock used for RTCRtpContributingSource.timestamp
Fixes #1690 and #1497.
Following the decision at the January VI, this timestamp uses
This will allow an application to tell "how long ago did media from
this source play?", whereas previously it was only defined as "a
local clock", which meant it only supported comparing timestamps