Mark public ObjC APIs as explicitly visible.

webrtc/api/objc/RTCVideoTrack.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 "webrtc/api/objc/RTCMediaStreamTrack.h"
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
 @protocol RTCVideoRenderer;
 @class RTCPeerConnectionFactory;
 @class RTCVideoSource;
 
+RTC_EXPORT
 @interface RTCVideoTrack : RTCMediaStreamTrack
 
 /** The video source for this video track. */
 @property(nonatomic, readonly) RTCVideoSource *source;
 
 - (instancetype)init NS_UNAVAILABLE;
 
 /** Register a renderer that will render all frames received on this track. */
 - (void)addRenderer:(id<RTCVideoRenderer>)renderer;
 
 /** Deregister a renderer. */
 - (void)removeRenderer:(id<RTCVideoRenderer>)renderer;
 
 @end
 
 NS_ASSUME_NONNULL_END