Update API for Objective-C RTCAudioTrack and RTCVideoTrack.

webrtc/api/objc/RTCAudioTrack.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 <Foundation/Foundation.h>
-
-#include <string>
+#import "RTCMediaStreamTrack.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface NSString (StdString)
+@interface RTCAudioTrack : RTCMediaStreamTrack
 
-@property(nonatomic, readonly) std::string stdString;
+- (instancetype)init NS_UNAVAILABLE;
 
-+ (std::string)stdStringForString:(NSString *)nsString;
-+ (NSString *)stringForStdString:(const std::string&)stdString;
+/** Initialize an RTCAudioTrack with an id. */
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+                        trackId:(NSString *)trackId;
 
 @end
 
 NS_ASSUME_NONNULL_END