MacOS: Add metal renderer and view.

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLI420Renderer.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>
+
+#import "WebRTC/RTCVideoFrame.h"
+
+/**
+ * Protocol defining ability to render RTCVideoFrame in Metal enabled views.
+ */
+@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;
+@end
+
+NS_AVAILABLE_MAC(10.11)
+/**
+ * Implementation of RTCMTLRenderer protocol for rendering native nv12 video frames.
+ */
+@interface RTCMTLI420Renderer : NSObject <RTCMTLRenderer>
+
+/**
+ * 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.
+ */
+- (BOOL)addRenderingDestination:(__kindof MTKView *)view;
+@end