Mark public ObjC APIs as explicitly visible.

webrtc/api/objc/RTCSessionDescription.h

 /*
  *  Copyright 2015 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"
+
 /**
  * Represents the session description type. This exposes the same types that are
  * in C++, which doesn't include the rollback type that is in the W3C spec.
  */
 typedef NS_ENUM(NSInteger, RTCSdpType) {
   RTCSdpTypeOffer,
   RTCSdpTypePrAnswer,
   RTCSdpTypeAnswer,
 };
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCSessionDescription : NSObject
 
 /** The type of session description. */
 @property(nonatomic, readonly) RTCSdpType type;
 
 /** The SDP string representation of this session description. */
 @property(nonatomic, readonly) NSString *sdp;
 
 - (instancetype)init NS_UNAVAILABLE;
 
 /** Initialize a session description with a type and SDP string. */
 - (instancetype)initWithType:(RTCSdpType)type sdp:(NSString *)sdp
     NS_DESIGNATED_INITIALIZER;
 
 + (NSString *)stringForType:(RTCSdpType)type;
 
 + (RTCSdpType)typeForString:(NSString *)string;
 
 @end
 
 NS_ASSUME_NONNULL_END