iOS: Add new RTCVideoSource interface

The new RTCVideoSource interface can be used by custom implementations of RTCVideoCapturer.

BUG=webrtc:7177
TBR=tommi

Review-Url: https://codereview.webrtc.org/2745193002
Cr-Commit-Position: refs/heads/master@{#17409}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
index ea603b9..a4eaefb 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
@@ -113,4 +113,8 @@
   return self;
 }
 
+- (rtc::scoped_refptr<webrtc::VideoFrameBuffer>)videoBuffer {
+  return _videoBuffer;
+}
+
 @end