Added the API to create an RTCRtpSender to the Objective C wrapper.

webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm

Index: webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm b/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm
index 3d307be507d30e1f474e40d2d17cb08a49fe6646..208550f6c866db47f6d73c8c56cfb2fb3eff6e2e 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm
@@ -12,6 +12,11 @@
 
 #import "NSString+StdString.h"
 
+NSString * const kRTCMediaStreamTrackKindAudio =
+    @(webrtc::MediaStreamTrackInterface::kAudioKind);
+NSString * const kRTCMediaStreamTrackKindVideo =
+    @(webrtc::MediaStreamTrackInterface::kVideoKind);
+
 @implementation RTCMediaStreamTrack {
   rtc::scoped_refptr<webrtc::MediaStreamTrackInterface> _nativeTrack;
   RTCMediaStreamTrackType _type;