iOS: Add new RTCVideoSource interface

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

 /*
  *  Copyright 2015 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.
  */
 
@@ skipping 95 matching lines @@
                            rotation:(RTCVideoRotation)rotation
                         timeStampNs:(int64_t)timeStampNs {
   if (self = [super init]) {
     _videoBuffer = videoBuffer;
     _rotation = rotation;
     _timeStampNs = timeStampNs;
   }
   return self;
 }
 
+- (rtc::scoped_refptr<webrtc::VideoFrameBuffer>)videoBuffer {
+  return _videoBuffer;
+}
+
 @end