Update API for Objective-C RTCMediaStreamTrack.

R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1527143002 .

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

Cr-Original-Commit-Position: refs/heads/master@{#11208}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 81028796bc5b359c44a0b4a9d85c55e5db41f833
5 files changed