Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/download_task_runner.h

Index: content/browser/download/download_task_runner.h
diff --git a/content/browser/download/download_task_runner.h b/content/browser/download/download_task_runner.h
new file mode 100644
index 0000000000000000000000000000000000000000..b8bb5a4bec6cd23e9028c240a9e14e1b3490f04d
--- /dev/null
+++ b/content/browser/download/download_task_runner.h
@@ -0,0 +1,18 @@
+// 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.
+
+#ifndef CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_TASK_RUNNER_H_
+#define CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_TASK_RUNNER_H_
+
+#include "base/sequenced_task_runner.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+// Returns the task runner used to save files and do other blocking operations.
+CONTENT_EXPORT scoped_refptr<base::SequencedTaskRunner> GetDownloadTaskRunner();
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_TASK_RUNNER_H_