Add a lock in ChunkDemuxer::SetStreamStatusChangeCB

This method accesses audio/video_streams_ collections and must acquire
the |lock_| since those collections might be modified from a different
thread.

BUG=726526

Change-Id: Ibb9ab848612f9bace1321aed56a4d22706ab7bea
Reviewed-on: https://chromium-review.googlesource.com/517226Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Matthew Wolenetz <wolenetz@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Commit-Queue: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476462}

media/filters/chunk_demuxer.cc

@@ -534,6 +534,7 @@ std::vector<DemuxerStream*> ChunkDemuxer::GetAllStreams() {
 }
 
 void ChunkDemuxer::SetStreamStatusChangeCB(const StreamStatusChangeCB& cb) {
+  base::AutoLock auto_lock(lock_);
   DCHECK(!cb.is_null());
   for (const auto& stream : audio_streams_)
     stream->SetStreamStatusChangeCB(cb);