MacOS: Add metal renderer and view.

BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2778693003
Cr-Commit-Position: refs/heads/master@{#17482}

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

@@ -0,0 +1,17 @@
+/*
+ *  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 <Cocoa/Cocoa.h>
+#import "WebRTC/RTCVideoRenderer.h"
+
+NS_AVAILABLE_MAC(10.11)
+@interface RTCMTLNSVideoView : NSView <RTCVideoRenderer>
+
+@end