Ignore sink start attempts when no frames are present.

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 38 matching lines @@
     base::ResetAndReturn(&stop_cb_).Run();
 }
 
 void NullVideoSink::CallRender() {
   DCHECK(task_runner_->BelongsToCurrentThread());
   DCHECK(started_);
 
   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())
     new_frame_cb_.Run(new_frame);
 
   current_render_time_ += interval_;
 
   if (clockless_) {
     task_runner_->PostTask(FROM_HERE, cancelable_worker_.callback());
     return;
@@ skipping 17 matching lines @@
   task_runner_->PostDelayedTask(FROM_HERE, cancelable_worker_.callback(),
                                 delay);
 }
 
 void NullVideoSink::PaintFrameUsingOldRenderingPath(
     const scoped_refptr<VideoFrame>& frame) {
   new_frame_cb_.Run(frame);
 }
 
 }  // namespace media