PSA: RtcEvent timestamping is changing

[Danil Chapovalov]

If you do not use RtcEventLog, or use RtcEventLog provided by WebRTC, no action is needed,

However, If you inject custom implementation of the RtcEventLog please read on.

RtcEvent contains timestamp field - time when event was logged. Historically that time was queried from the global clock in the RtcEvent constructor.

Responsibility to populate the timestamp is moving into RtcEventLog implementation starting in https://webrtc-review.googlesource.com/c/src/+/412300

This is done to allow use of injectable clock for RtcEvent logging.

RtcEvent still populates current time from the global clock, however in the future that will be removed requiring all RtcEventLog implementations to record time of the events.