Persist RTP state for FlexFEC.

Before this CL, the RTP state would be re-randomized after a
recreation of VideoSendStream. That might lead to us sending
a non-compliant RTP stream, which is avoided after the
changes in this CL.

BUG=webrtc:5654
TBR=pbos@webrtc.org  # Trivial change to fuzzer.

Review-Url: https://codereview.webrtc.org/2912713002
Cr-Original-Commit-Position: refs/heads/master@{#18322}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 48d21a23c6663dc1d07c8a05f2922ad14bd12c8a
7 files changed