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}