New function AbsSequenceNumberDifference. Bug: None Change-Id: I3906e3be313ec69973a20096c17c06e20448149d Reviewed-on: https://webrtc-review.googlesource.com/4384 Commit-Queue: Stefan Holmer <stefan@webrtc.org> Reviewed-by: Stefan Holmer <stefan@webrtc.org> Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20086}