Update BufferedDataSource class documentation.

media/blink/buffered_data_source.cc

Index: media/blink/buffered_data_source.cc
diff --git a/media/blink/buffered_data_source.cc b/media/blink/buffered_data_source.cc
index 16128f1dd307a44198d2fbb1fcdfda95d16820ac..0eb52dd220bcf17ce62062f7a18adecc8eb4cada 100644
--- a/media/blink/buffered_data_source.cc
+++ b/media/blink/buffered_data_source.cc
@@ -104,9 +104,12 @@ BufferedDataSource::BufferedDataSource(
       weak_factory_(this) {
   DCHECK(host_);
   DCHECK(!downloading_cb_.is_null());
+  DCHECK(render_task_runner_->BelongsToCurrentThread());
 }
 
-BufferedDataSource::~BufferedDataSource() {}
+BufferedDataSource::~BufferedDataSource() {
+  DCHECK(render_task_runner_->BelongsToCurrentThread());
+}
 
 // A factory method to create BufferedResourceLoader using the read parameters.
 // This method can be overridden to inject mock BufferedResourceLoader object