This CL introduces a way for the VideoReceiveStreams to check whether they are protected by any FlexfecReceiveStreams. This is done in the VideoReceiveStream::Start() method, which then propagates this information down to the jitter buffer adaptation logic. BUG=webrtc:5654 Review-Url: https://codereview.webrtc.org/2649973005 Cr-Commit-Position: refs/heads/master@{#16328}