This is a preparation for extracting CSRC book-keeping to its own class. Bug: webrtc:7135 Change-Id: Ic51ceb57ec53a43064a3d0392de8baa978a4e8cf Reviewed-on: https://webrtc-review.googlesource.com/93463 Reviewed-by: Danil Chapovalov <danilchap@webrtc.org> Commit-Queue: Niels Moller <nisse@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24257}