CHECK that media::PipelineImpl::RendererWrapper is given real renderer.

Trying to track down causes of the crash (initializing a null
renderer).

Bug: 827990
Change-Id: I2fe20dd55921eaf43fccb89a69ee9f9135443b4f
Reviewed-on: https://chromium-review.googlesource.com/994142Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547948}

media/base/pipeline_impl.cc

@@ -229,6 +229,9 @@ void PipelineImpl::RendererWrapper::Start(
   DCHECK(state_ == kCreated || state_ == kStopped)
       << "Received start in unexpected state: " << state_;
 
+  // Tracking down http://crbug.com/827990
+  CHECK(renderer);
+
   SetState(kStarting);
 
   DCHECK(!demuxer_);
@@ -400,6 +403,9 @@ void PipelineImpl::RendererWrapper::Resume(std::unique_ptr<Renderer> renderer,
                                            base::TimeDelta timestamp) {
   DCHECK(media_task_runner_->BelongsToCurrentThread());
 
+  // Tracking down http://crbug.com/827990
+  CHECK(renderer);
+
   // Suppress resuming if we're not suspended.
   if (state_ != kSuspended) {
     DCHECK(state_ == kStopping || state_ == kStopped)