Add new PeerConnection APIs to the ObjC SDK

This CL adds wrappers for the following PeerConnection native
APIs to the Objective C API:
- SdpSemantics enum added to the RTCConfiguration
- RTCRtpTransceiver
- RTCPeerConnection.addTrack
- RTCPeerConnection.removeTrack
- RTCPeerConnection.addTransceiver
- RTCPeerConnection.transceivers

Bug: webrtc:8870
Change-Id: I9449df9742a59e90894712dc7749ca30b569d94b
Reviewed-on: https://webrtc-review.googlesource.com/54780
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22214}

sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm

@@ -126,4 +126,26 @@ void RtpReceiverDelegateAdapter::OnFirstPacketReceived(
   }
 }
 
++ (cricket::MediaType)nativeMediaTypeForMediaType:(RTCRtpMediaType)mediaType {
+  switch (mediaType) {
+    case RTCRtpMediaTypeAudio:
+      return cricket::MEDIA_TYPE_AUDIO;
+    case RTCRtpMediaTypeVideo:
+      return cricket::MEDIA_TYPE_VIDEO;
+    case RTCRtpMediaTypeData:
+      return cricket::MEDIA_TYPE_DATA;
+  }
+}
+
++ (NSString *)stringForMediaType:(RTCRtpMediaType)mediaType {
+  switch (mediaType) {
+    case RTCRtpMediaTypeAudio:
+      return @"AUDIO";
+    case RTCRtpMediaTypeVideo:
+      return @"VIDEO";
+    case RTCRtpMediaTypeData:
+      return @"DATA";
+  }
+}
+
 @end