Add parsing/serializing for MID RTP header extension.

This is the first in a series of CLs to add support for media
identification as part of unified plan SDP.

Bug: webrtc:4050
Change-Id: I0eb5639d240a9a1412c2b047a33d5112e4901f26
Reviewed-on: https://chromium-review.googlesource.com/576374
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19111}