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

Objective C applications can now create new RTCRtpSenders and change their tracks, which gives them more fine grained control than MediaStreams.

BUG=

Review-Url: https://codereview.webrtc.org/1888633002
Cr-Commit-Position: refs/heads/master@{#12570}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm b/webrtc/sdk/objc/Framework/Classes/RTCMediaStreamTrack.mm
index 3d307be..208550f 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;