Use the last video stream for scenario tests stats.

This makes slightly more sense when looking at video resolution etc.

Bug: webrtc:9510
Change-Id: I49d39cac23d2f5d7ca09f2a27152c7519ea639f1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169344
Reviewed-by: Jakob Ivarsson <jakobi@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30632}

test/scenario/video_stream.cc

@@ -615,7 +615,7 @@ VideoReceiveStream::Stats ReceiveVideoStream::GetStats() const {
   if (receive_streams_.empty())
     return VideoReceiveStream::Stats();
   // TODO(srte): Handle multiple receive streams.
-  return receive_streams_.front()->GetStats();
+  return receive_streams_.back()->GetStats();
 }
 
 VideoStreamPair::~VideoStreamPair() = default;