Mark public ObjC APIs as explicitly visible.

webrtc/base/objc/RTCCameraPreviewView.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 <UIKit/UIKit.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 @class AVCaptureSession;
 @class RTCAVFoundationVideoSource;
 
 /** RTCCameraPreviewView is a view that renders local video from an
  *  AVCaptureSession.
  */
+RTC_EXPORT
 @interface RTCCameraPreviewView : UIView
 
 /** The capture session being rendered in the view. Capture session
  *  is assigned to AVCaptureVideoPreviewLayer async in the same
  *  queue that the AVCaptureSession is started/stopped.
  */
 @property(nonatomic, strong) AVCaptureSession *captureSession;
 
 @end