Update API for Objective-C RTCAudioTrack and RTCVideoTrack.

webrtc/api/objc/RTCMediaStreamTrack+Private.h

 /*
  *  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.
  */
 
 #import "RTCMediaStreamTrack.h"
 
 #include "talk/app/webrtc/mediastreaminterface.h"
 #include "webrtc/base/scoped_ptr.h"
 
+typedef NS_ENUM(NSInteger, RTCMediaStreamTrackType) {
+  RTCMediaStreamTrackTypeAudio,
+  RTCMediaStreamTrackTypeVideo,
+};
+
 NS_ASSUME_NONNULL_BEGIN
 
 @interface RTCMediaStreamTrack ()
 
 /**
- * The native MediaStreamTrackInterface representation of this
- * RTCMediaStreamTrack object. This is needed to pass to the underlying C++
- * APIs.
+ * The native MediaStreamTrackInterface passed in or created during
+ * construction.
  */
 @property(nonatomic, readonly)
     rtc::scoped_refptr<webrtc::MediaStreamTrackInterface> nativeTrack;
 
 /**
  * Initialize an RTCMediaStreamTrack from a native MediaStreamTrackInterface.
  */
 - (instancetype)initWithNativeTrack:
     (rtc::scoped_refptr<webrtc::MediaStreamTrackInterface>)nativeTrack
+                               type:(RTCMediaStreamTrackType)type
     NS_DESIGNATED_INITIALIZER;
 
 + (webrtc::MediaStreamTrackInterface::TrackState)nativeTrackStateForState:
     (RTCMediaStreamTrackState)state;
 
 + (RTCMediaStreamTrackState)trackStateForNativeState:
     (webrtc::MediaStreamTrackInterface::TrackState)nativeState;
 
 + (NSString *)stringForState:(RTCMediaStreamTrackState)state;
 
 @end
 
 NS_ASSUME_NONNULL_END