Clean up FlexfecReceiveStream ctor signatures.

BUG=webrtc:5654

Review-Url: https://codereview.webrtc.org/2535173008
Cr-Original-Commit-Position: refs/heads/master@{#15476}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 446fcb6cadb90fb47bc72053f29994ae56c80d1f
6 files changed