Do not use NullVideoSink::new_frame_cb_ if NULL.

media/base/null_video_sink.cc

Index: media/base/null_video_sink.cc
diff --git a/media/base/null_video_sink.cc b/media/base/null_video_sink.cc
index 6e03e53eaa0fe69428ef2c2f43340754563b0983..7a5c54da09a1d3852bbc76ac2bfc1ad2c8ac09a3 100644
--- a/media/base/null_video_sink.cc
+++ b/media/base/null_video_sink.cc
@@ -92,8 +92,10 @@ void NullVideoSink::PaintSingleFrame(const scoped_refptr<VideoFrame>& frame,
                                      bool repaint_duplicate_frame) {
   if (!repaint_duplicate_frame && frame == last_frame_)
     return;
+
   last_frame_ = frame;
-  new_frame_cb_.Run(frame);
+  if (!new_frame_cb_.is_null())
+    new_frame_cb_.Run(frame);
 }
 
 }  // namespace media