Explain the reason why we don't have thread check in destructor of ByteStreamReader/WriterImpl

content/browser/download/download_resource_handler.cc

Index: content/browser/download/download_resource_handler.cc
diff --git a/content/browser/download/download_resource_handler.cc b/content/browser/download/download_resource_handler.cc
index c651ed9acdbcf8d470f6670cbf0876fb98e2d653..ab9e19ef3df08ba3c7f79af4dce81caa8c5143c1 100644
--- a/content/browser/download/download_resource_handler.cc
+++ b/content/browser/download/download_resource_handler.cc
@@ -59,6 +59,10 @@ static void StartOnUIThread(
     // download.
     if (!started_cb.is_null())
       started_cb.Run(NULL, DOWNLOAD_INTERRUPT_REASON_USER_CANCELED);
+
+    // |stream| gets deleted on non-FILE thread, but it's ok since
+    // we're not using stream_writer_ yet.
+
     return;
   }