Fix race in EndToEndTest.ReceivesFlexfecAndSendsCorrespondingRtcp.
Fix race in EndToEndTest.ReceivesFlexfecAndSendsCorrespondingRtcp.

R=stefan@webrtc.org
BUG=webrtc:7004

Review-Url: https://codereview.webrtc.org/2639173002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16136}

ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc