Mark public ObjC APIs as explicitly visible.

webrtc/api/objc/RTCMediaStreamTrack.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>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 /**
  * Represents the state of the track. This exposes the same states in C++.
  */
 typedef NS_ENUM(NSInteger, RTCMediaStreamTrackState) {
   RTCMediaStreamTrackStateLive,
   RTCMediaStreamTrackStateEnded
 };
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCMediaStreamTrack : NSObject
 
 /**
  * The kind of track. For example, "audio" if this track represents an audio
  * track and "video" if this track represents a video track.
  */
 @property(nonatomic, readonly) NSString *kind;
 
 /** An identifier string. */
 @property(nonatomic, readonly) NSString *trackId;
 
 /** The enabled state of the track. */
 @property(nonatomic, assign) BOOL isEnabled;
 
 /** The state of the track. */
 @property(nonatomic, readonly) RTCMediaStreamTrackState readyState;
 
 - (instancetype)init NS_UNAVAILABLE;
 
 @end
 
 NS_ASSUME_NONNULL_END