Update API for Objective-C RTCSessionDescription.

BUG=
R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1524303006 .

Patch from Jon Hjelle <hjon@andyet.net>.

Cr-Original-Commit-Position: refs/heads/master@{#11157}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 67e83d653968f1147f7cb86451c4f4494751643a
7 files changed