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/RTCMediaConstraints.h b/webrtc/api/objc/RTCMediaConstraints.h
index a8ad391..d726f7d 100644
--- a/webrtc/api/objc/RTCMediaConstraints.h
+++ b/webrtc/api/objc/RTCMediaConstraints.h
@@ -10,8 +10,11 @@
 
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCMediaConstraints : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;