Media type is not part of the WebRTC spec for RtpTransceiver, but it is
handy and the RtpSender/RtpReceiver also have it.
Bug: webrtc:7600
Change-Id: I8350069502588bff478db4dc1318329626dcf9be
Reviewed-on: https://webrtc-review.googlesource.com/50560
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21988}