Fix setting of recovered flag in RtxReceiveStream.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/3005793002
Cr-Original-Commit-Position: refs/heads/master@{#19599}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 386449971a8f471e87f3844ebee1b1bc74144998