Ignoring retransmissions carried over the RTX stream was a bug. This CL fixes the bug, so that all retransmissions are accounted for. It also adds test coverage for this. This resolves https://crbug.com/webrtc/11440 but does not resolve https://crbug.com/webrtc/11439. Bug: webrtc:11440, webrtc:11439 Change-Id: Ifb10aa60a0f453738aaa30de90eaa5b31f9ec265 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170639 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Henrik Boström <hbos@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30822}