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

content/browser/download/download_resource_handler.cc

 // Copyright (c) 2012 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 "content/browser/download/download_resource_handler.h"
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/logging.h"
@@ skipping 41 matching lines @@
     scoped_ptr<ByteStreamReader> stream,
     const DownloadUrlParameters::OnStartedCallback& started_cb) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   DownloadManager* download_manager = info->request_handle.GetDownloadManager();
   if (!download_manager) {
     // NULL in unittests or if the page closed right after starting the
     // 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;
   }
 
   download_manager->StartDownload(info.Pass(), stream.Pass(), started_cb);
 }
 
 }  // namespace
 
 const int DownloadResourceHandler::kDownloadByteStreamSize = 100 * 1024;
 
@@ skipping 404 matching lines @@
 
   // Remove output stream callback if a stream exists.
   if (stream_writer_)
     stream_writer_->RegisterCallback(base::Closure());
 
   UMA_HISTOGRAM_TIMES("SB2.DownloadDuration",
                       base::TimeTicks::Now() - download_start_time_);
 }
 
 }  // namespace content