Parsing of transport wide sequence number rtp extension header.
Plus some refactoring to correctly handle padding.

BUG=4311
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45429004

Cr-Original-Commit-Position: refs/heads/master@{#8757}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 30933904797ab220a7a1532a535904f9d8ee3149
10 files changed