Add metal view, shaders and renderer.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.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 "WebRTC/RTCVideoRenderer.h"
+
+NS_ASSUME_NONNULL_BEGIN
+RTC_EXPORT
+
+/**
+ * RTCMTLVideoView is thin wrapper around MTKView.
+ *
+ * It has id<RTCVideoRenderer> property that renders video frames in the view's
+ * bounds using Metal.
+ */
+NS_CLASS_AVAILABLE_IOS(9)
+@interface RTCMTLVideoView : UIView <RTCVideoRenderer>
+
+@end
+NS_ASSUME_NONNULL_END