ObjC: RTCVideoSource cleanup

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoCapturer.h

 /*
  *  Copyright 2017 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 <WebRTC/RTCVideoFrame.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
 @class RTCVideoCapturer;
 
 RTC_EXPORT
-
-@protocol RTCVideoCapturerDelegate
+@protocol RTCVideoCapturerDelegate <NSObject>
 - (void)capturer:(RTCVideoCapturer *)capturer didCaptureVideoFrame:(RTCVideoFrame *)frame;
 @end
 
+RTC_EXPORT
 @interface RTCVideoCapturer : NSObject
+
+@property(nonatomic, readonly, weak) id<RTCVideoCapturerDelegate> delegate;
+
 - (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate;
-@property(nonatomic, readonly, weak) id<RTCVideoCapturerDelegate> delegate;
+
 @end
 
 NS_ASSUME_NONNULL_END