Mark public ObjC APIs as explicitly visible.

BUG=

Review URL: https://codereview.webrtc.org/1899573002

Cr-Commit-Position: refs/heads/master@{#12431}
diff --git a/webrtc/api/objc/RTCMediaStream.h b/webrtc/api/objc/RTCMediaStream.h
index 50ae7df..d5de6d3 100644
--- a/webrtc/api/objc/RTCMediaStream.h
+++ b/webrtc/api/objc/RTCMediaStream.h
@@ -10,12 +10,15 @@
 
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
 @class RTCAudioTrack;
 @class RTCPeerConnectionFactory;
 @class RTCVideoTrack;
 
+RTC_EXPORT
 @interface RTCMediaStream : NSObject
 
 /** The audio tracks in this stream. */