Add FEC producer fuzzing and a unittest for one of the issues found.

BUG=webrtc:4800
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1522463002 .

Cr-Original-Commit-Position: refs/heads/master@{#10990}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4c1093b86f4d0a1c8ade68a4b6a411b2674deac8
4 files changed