ObjC: RTCVideoSource cleanup

RTCVideoSource was recently added in
https://codereview.webrtc.org/2745193002/. This CL addresses some post
commit feedback.

BUG=webrtc:7177

Review-Url: https://codereview.webrtc.org/2812533003
Cr-Commit-Position: refs/heads/master@{#17649}

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

@@ -15,14 +15,17 @@ 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
-- (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate;
+
 @property(nonatomic, readonly, weak) id<RTCVideoCapturerDelegate> delegate;
+
+- (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate;
+
 @end
 
 NS_ASSUME_NONNULL_END