Do not use NullVideoSink::new_frame_cb_ if NULL.

Review-Url: https://codereview.chromium.org/2282963002
Cr-Commit-Position: refs/heads/master@{#414852}

media/base/null_video_sink.cc

@@ -92,7 +92,9 @@ void NullVideoSink::PaintSingleFrame(const scoped_refptr<VideoFrame>& frame,
                                      bool repaint_duplicate_frame) {
   if (!repaint_duplicate_frame && frame == last_frame_)
     return;
+
   last_frame_ = frame;
+  if (!new_frame_cb_.is_null())
     new_frame_cb_.Run(frame);
 }