Ignore sink start attempts when no frames are present.

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 0d9746d3620af61fcf3ffba3a4b50982c026e024..f8471056c99c85f6b9f185d5c046adbe05ade819 100644
--- a/media/base/null_video_sink.cc
+++ b/media/base/null_video_sink.cc
@@ -56,6 +56,7 @@ void NullVideoSink::CallRender() {
   const base::TimeTicks end_of_interval = current_render_time_ + interval_;
   scoped_refptr<VideoFrame> new_frame = callback_->Render(
       current_render_time_, end_of_interval, background_render_);
+  DCHECK(new_frame);
   const bool is_new_frame = new_frame != last_frame_;
   last_frame_ = new_frame;
   if (is_new_frame && !new_frame_cb_.is_null())