Set local SSRCs on receivers added before senders.

Addresses bug where a receiver would report SSRC 1 even though the
endpoint has sending streams.

BUG=4678
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51099004

Cr-Commit-Position: refs/heads/master@{#9262}
3 files changed