Mark public ObjC APIs as explicitly visible.

webrtc/base/objc/RTCTracing.h

Index: webrtc/base/objc/RTCTracing.h
diff --git a/webrtc/base/objc/RTCTracing.h b/webrtc/base/objc/RTCTracing.h
index 5b8e2f6ac371abe543149a65f54b0d04983ed7cb..6f0596cfb711dbb8ac45f20985779da21d3c62f7 100644
--- a/webrtc/base/objc/RTCTracing.h
+++ b/webrtc/base/objc/RTCTracing.h
@@ -12,10 +12,10 @@
 
 #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();