Get rid of unnecessary cast of FlexfecReceiveStreamImpl to FlexfecReceiveStream

BUG=None

Review-Url: https://codereview.webrtc.org/2967913002
Cr-Original-Commit-Position: refs/heads/master@{#19131}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 42f44f9cf672eb4bc43616c11e625111f48b6366
5 files changed