iOS: Add new RTCVideoSource interface

webrtc/sdk/objc/Framework/Classes/RTCVideoCapturer.mm

+/*
+ *  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/RTCVideoCapturer.h"
+
+@implementation RTCVideoCapturer {
+  __weak id<RTCVideoCapturerDelegate> _delegate;
+}
+
+- (instancetype)initWithDelegate:(__weak id<RTCVideoCapturerDelegate>)delegate {
+  if (self = [super init]) {
+    _delegate = delegate;
+  }
+  return self;
+}
+
+- (id<RTCVideoCapturerDelegate>)delegate {
+  return _delegate;
+}
+
+@end