Add GUARDED_BY's in FlexfecReceiver.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2589583004
Cr-Original-Commit-Position: refs/heads/master@{#15688}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8b5c345ee55e390c7e78241e5924c6674dc5aa3f
2 files changed