Add skeleton webrtc::SessionDescription and webrtc::MediaDescription classes.

BUG=webrtc:7311

Review-Url: https://codereview.webrtc.org/2743003004
Cr-Original-Commit-Position: refs/heads/master@{#17181}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 55adc0e1a5f39f7b4bb706c1cbc688e75c9754ae
7 files changed