Mark public ObjC APIs as explicitly visible.

webrtc/base/objc/RTCTracing.h

 /*
  *  Copyright 2016 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"
 
-RTC_EXPORT void RTCSetupInternalTracer();
+RTC_EXTERN void RTCSetupInternalTracer();
 /** Starts capture to specified file. Must be a valid writable path.
  *  Returns YES if capture starts.
  */
-RTC_EXPORT BOOL RTCStartInternalCapture(NSString *filePath);
-RTC_EXPORT void RTCStopInternalCapture();
-RTC_EXPORT void RTCShutdownInternalTracer();
+RTC_EXTERN BOOL RTCStartInternalCapture(NSString *filePath);
+RTC_EXTERN void RTCStopInternalCapture();
+RTC_EXTERN void RTCShutdownInternalTracer();