Make VideoReceiveStream::GetStats() const.

BUG=
R=mflodman@webrtc.org, pbos@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/8169004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5501 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/video_receive_stream.h b/video_receive_stream.h
index f1eea82..febb221 100644
--- a/video_receive_stream.h
+++ b/video_receive_stream.h
@@ -173,7 +173,7 @@
 
   virtual void StartReceiving() = 0;
   virtual void StopReceiving() = 0;
-  virtual Stats GetStats() = 0;
+  virtual Stats GetStats() const = 0;
 
   // TODO(mflodman) Replace this with callback.
   virtual void GetCurrentReceiveCodec(VideoCodec* receive_codec) = 0;