The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36

commit ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36
Author: Henrik Boström <hbos@chromium.org>
Date: Thu Jan 10 13:05:13 2019

RTCRtpReceiver.getSynchronizationSources() added.

This CL:
- Adds getSynchronizationSources() for both audio and video.
- Adds proper test coverage, including test coverage for unsupported
  members audioLevel and voiceActivityFlag. It turns out that our
  timestamps are not comparable to performance.now(), this should be
  fixed separately (this must already have been the case for
  getContributingSources() but not noticed until now).

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/wTJMXOwcV1A/uszhuWsGCQAJ

Bug:  893158 ,  893160 
Change-Id: Ic90344af7fa48c3cd0b94929cc9453deb9dc7f9f
Reviewed-on: https://chromium-review.googlesource.com/c/1401068
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621558}
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/renderer/modules/modules_idl_files.gni
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/renderer/modules/peerconnection/rtc_rtp_receiver.cc
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/renderer/modules/peerconnection/rtc_rtp_receiver.h
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/renderer/modules/peerconnection/rtc_rtp_receiver.idl
[add] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/renderer/modules/peerconnection/rtc_rtp_synchronization_source.idl
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/external/wpt/webrtc/RTCRtpReceiver-getSynchronizationSources.https-expected.txt
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/external/wpt/webrtc/RTCRtpReceiver-getSynchronizationSources.https.html
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/external/wpt/webrtc/idlharness.https.window-expected.txt
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/idlharness.https.window-expected.txt
[modify] https://crrev.com/ea54e65cc79e2ec87ce021f9acbdd2d51b1f2a36/third_party/blink/web_tests/webexposed/global-interface-listing-expected.txt