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

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

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 293 matching lines @@
 }
 
 - (void)setRemoteDescription:(RTCSessionDescription *)sdp
            completionHandler:(void (^)(NSError *error))completionHandler {
   rtc::scoped_refptr<webrtc::SetSessionDescriptionObserverAdapter> observer(
       new rtc::RefCountedObject<webrtc::SetSessionDescriptionObserverAdapter>(
           completionHandler));
   _peerConnection->SetRemoteDescription(observer, sdp.nativeDescription);
 }
 
+- (RTCRtpSender *)senderWithKind:(NSString *)kind
+                        streamId:(NSString *)streamId {
+  std::string nativeKind = [NSString stdStringForString:kind];
+  std::string nativeStreamId = [NSString stdStringForString:streamId];
+  rtc::scoped_refptr<webrtc::RtpSenderInterface> nativeSender(
+      _peerConnection->CreateSender(nativeKind, nativeStreamId));
+  return nativeSender ?
+      [[RTCRtpSender alloc] initWithNativeRtpSender:nativeSender]
+      : nil;
+}
+
 - (NSArray<RTCRtpSender *> *)senders {
   std::vector<rtc::scoped_refptr<webrtc::RtpSenderInterface>> nativeSenders(
       _peerConnection->GetSenders());
   NSMutableArray *senders = [[NSMutableArray alloc] init];
   for (const auto &nativeSender : nativeSenders) {
     RTCRtpSender *sender =
         [[RTCRtpSender alloc] initWithNativeRtpSender:nativeSender];
     [senders addObject:sender];
   }
   return senders;
@@ skipping 162 matching lines @@
     case RTCStatsOutputLevelDebug:
       return webrtc::PeerConnectionInterface::kStatsOutputLevelDebug;
   }
 }
 
 - (rtc::scoped_refptr<webrtc::PeerConnectionInterface>)nativePeerConnection {
   return _peerConnection;
 }
 
 @end