Add RTCFileVideoCapturer class.

webrtc/sdk/objc/Framework/Classes/RTCFileVideoCapturer.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/RTCVideoCapturer.h>
+
+/**
+ * RTCVideoCapturer that reads buffers from file.
+ *
+ * Per design, the file capturer can only be run once and once stopped it cannot run again.
+ * To run another file capture session, create new instance of the class.
+ */
+@interface RTCFileVideoCapturer : RTCVideoCapturer
+
+- (void)startCapturingFromFileNamed:(NSString *)nameOfFile;
+- (void)stopCapture;
+
+@end