Mark public ObjC APIs as explicitly visible.

webrtc/base/objc/RTCFieldTrials.h

Index: webrtc/base/objc/RTCFieldTrials.h
diff --git a/webrtc/base/objc/RTCFieldTrials.h b/webrtc/base/objc/RTCFieldTrials.h
index 837a8f7a60993f7c172bef9e342e8a30e4432bea..52e90fed3a2ec7ad966bcd9063d7a6f5a15f1aa1 100644
--- a/webrtc/base/objc/RTCFieldTrials.h
+++ b/webrtc/base/objc/RTCFieldTrials.h
@@ -8,10 +8,10 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "webrtc/base/objc/RTCMacros.h"
-
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 typedef NS_OPTIONS(NSUInteger, RTCFieldTrialOptions) {
   RTCFieldTrialOptionsNone = 0,
   RTCFieldTrialOptionsSendSideBwe = 1 << 0,
@@ -20,4 +20,4 @@ typedef NS_OPTIONS(NSUInteger, RTCFieldTrialOptions) {
 /** Must be called before any other call into WebRTC. See:
  *  webrtc/system_wrappers/include/field_trial_default.h
  */
-RTC_EXPORT void RTCInitFieldTrials(RTCFieldTrialOptions options);
+RTC_EXTERN void RTCInitFieldTrials(RTCFieldTrialOptions options);