This change adds the writing and parsing of the `abs-capture-time` RTP header extension defined at: http://www.webrtc.org/experiments/rtp-hdrext/abs-capture-time We are still missing the code to: - Negotiate the header extension. - Collect capture time for audio and video and have the info sent with the header extension. - Receive the header extension and use its info. Bug: webrtc:10739 Change-Id: I75af492e994367f45a5bdc110af199900327b126 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144221 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Commit-Queue: Chen Xing <chxg@google.com> Cr-Commit-Position: refs/heads/master@{#28468}