iOS/MacOS:Refactor metal rendering by extracting common implementation

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLRenderer.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 <Foundation/Foundation.h>
-#import <MetalKit/MTKView.h>
+#if TARGET_OS_IPHONE
+#import <UIKit/UIKit.h>
+#else
+#import <AppKit/AppKit.h>
+#endif
 
 #import "WebRTC/RTCVideoFrame.h"
 
+NS_ASSUME_NONNULL_BEGIN
 /**
  * Protocol defining ability to render RTCVideoFrame in Metal enabled views.
  */
-@protocol RTCMTLRenderer <NSObject>
+@protocol RTCMTLRenderer<NSObject>
 
 /**
  * Method to be implemented to perform actual rendering of the provided frame.
  *
  * @param frame The frame to be rendered.
  */
 - (void)drawFrame:(RTCVideoFrame *)frame;
 
 /**
  * Sets the provided view as rendering destination if possible.
  *
  * If not possible method returns NO and callers of the method are responisble for performing
  * cleanups.
  */
+
+#if TARGET_OS_IOS
 - (BOOL)addRenderingDestination:(__kindof UIView *)view;
+#else
+- (BOOL)addRenderingDestination:(__kindof NSView *)view;
+#endif
 
 @end
 
 /**
  * Implementation of RTCMTLRenderer protocol for rendering native nv12 video frames.
  */
-@interface RTCMTLNV12Renderer : NSObject <RTCMTLRenderer>
+NS_AVAILABLE(10_11, 9_0)
+@interface RTCMTLRenderer : NSObject<RTCMTLRenderer>
+@end
 
-@end
+NS_ASSUME_NONNULL_END