This prepares for moving TransportFeedbackAdapter to TaskQueue. Bug: webrtc:9883 Change-Id: Ib333f6a6837ff6dd8b10813e8953e4d8cd5d8633 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/162040 Reviewed-by: Erik Språng <sprang@webrtc.org> Commit-Queue: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30076}