Downloads: replace BrowserThread::FILE with task scheduler.

content/public/browser/download_manager.cc

Index: content/public/browser/download_manager.cc
diff --git a/content/public/browser/download_manager.cc b/content/public/browser/download_manager.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c303d7a274a828f6ccae614a9f00abbcc05ef627
--- /dev/null
+++ b/content/public/browser/download_manager.cc
@@ -0,0 +1,16 @@
+// Copyright 2017 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/public/browser/download_manager.h"
+
+#include "content/browser/download/download_task_runner.h"
+
+namespace content {
+
+// static
+scoped_refptr<base::SequencedTaskRunner> DownloadManager::GetTaskRunner() {
+  return GetDownloadTaskRunner();
+}
+
+}  // namespace content