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/RTCRtpTransceiver+Private.h

@@ -0,0 +1,41 @@
+/*
+ *  Copyright 2018 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.
+ */
+
+#import "WebRTC/RTCRtpTransceiver.h"
+
+#include "api/rtptransceiverinterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCRtpTransceiverInit ()
+
+@property(nonatomic, readonly) webrtc::RtpTransceiverInit nativeInit;
+
+@end
+
+@interface RTCRtpTransceiver ()
+
+@property(nonatomic, readonly) rtc::scoped_refptr<webrtc::RtpTransceiverInterface>
+    nativeRtpTransceiver;
+
+/** Initialize an RTCRtpTransceiver with a native RtpTransceiverInterface. */
+- (instancetype)initWithNativeRtpTransceiver:
+        (rtc::scoped_refptr<webrtc::RtpTransceiverInterface>)nativeRtpTransceiver
+    NS_DESIGNATED_INITIALIZER;
+
++ (webrtc::RtpTransceiverDirection)nativeRtpTransceiverDirectionFromDirection:
+        (RTCRtpTransceiverDirection)direction;
+
++ (RTCRtpTransceiverDirection)rtpTransceiverDirectionFromNativeDirection:
+        (webrtc::RtpTransceiverDirection)nativeDirection;
+
+@end
+
+NS_ASSUME_NONNULL_END