This fixes an issue where SendSideCongestionControllerTest.OldFeedback
calls a function that posts a task on a TaskQueue and immediately after
changes the mocked observer that is called from that task.
Bug: webrtc:10056
Change-Id: Ib1cca5bf695482e75106bfc715662e4f76c381d9
Reviewed-on: https://webrtc-review.googlesource.com/c/112940
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25882}