Update RTCStatsCollector to work with RtpTransceivers

Bug: webrtc:8764
Change-Id: I8b442345869eb6d8b65fd12241ed7cb6e7d7ce3d
Reviewed-on: https://webrtc-review.googlesource.com/49580
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22026}

api/rtpreceiverinterface.cc

@@ -41,8 +41,4 @@ std::vector<RtpSource> RtpReceiverInterface::GetSources() const {
   return {};
 }
 
-int RtpReceiverInterface::AttachmentId() const {
-  return 0;
-}
-
 }  // namespace webrtc