Add the OnAddTrack callback for Objective-C wrapper.
Created an Obj-C wrapper for the callback OnAddTrack in this CL since it has been added to native C++ API
The callback function is called when a track is signaled by remote side and a new RtpReceiver is created.
The application can tell when tracks are added to the streams by listening to this callback.
BUG=webrtc:6112
Review-Url: https://codereview.webrtc.org/2513063003
Cr-Original-Commit-Position: refs/heads/master@{#16835}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 633f6fe0046131ed815098298b9a3120bac1d7a0
3 files changed
