iOS: Add new RTCVideoSource interface

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoCapturer.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 <WebRTC/RTCVideoFrame.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+RTC_EXPORT
+
+@protocol RTCVideoCapturerDelegate;
+
+@interface RTCVideoCapturer : NSObject
+- (instancetype)initWithDelegate:(__weak id<RTCVideoCapturerDelegate>)delegate;
+@property(nonatomic, readonly) id<RTCVideoCapturerDelegate> delegate;
+@end
+
+@protocol RTCVideoCapturerDelegate
+- (void)capturer:(RTCVideoCapturer*)capturer
+    didCaptureVideoFrame:(RTCVideoFrame*)frame;
+@end
+
+NS_ASSUME_NONNULL_END