This change adds a new SourceTracker class that can do spec-compliant tracking of RTCRtpContributingSource and RTCRtpSynchronizationSource when frames are delivered to the RTCRtpReceiver's MediaStreamTrack for playout. It will replace the existing spec-incompliant ContributingSources.
Bug: webrtc:10545 webrtc:10668
Change-Id: I961adaba09d6337f2f36b301a4fabcd20de65271
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140948
Commit-Queue: Chen Xing <chxg@google.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28249}
