Do not use NullVideoSink::new_frame_cb_ if NULL.

media/base/null_video_sink.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/base/null_video_sink.h"
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/location.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 74 matching lines @@
   }
 
   task_runner_->PostDelayedTask(FROM_HERE, cancelable_worker_.callback(),
                                 delay);
 }
 
 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