The definition of this field in RFC 3550 says that under certain conditions it may have a negative value. This change exposes that property in the WebRTC API. Bug: webrtc:8626 Change-Id: I4ee249da045dcee940db66ebd915268a97fc13db Reviewed-on: https://webrtc-review.googlesource.com/31260 Reviewed-by: Stefan Holmer <stefan@webrtc.org> Commit-Queue: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/master@{#21159}