Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/download_task_runner.h

+// Copyright (c) 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/single_thread_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::SingleThreadTaskRunner>

[gab, 2017/06/21 19:42:24]

What's the reason this couldn't be a SequencedTaskRunner again? We fixed many of
those recently so maybe it's fine now? If not, add a TODO with a bug flagged as
a blocker for https://crbug.com/675631 which explains what prevents this today.

[Sigurður Ásgeirsson, 2017/06/22 14:50:09]

There was something that barfed on base::DelayTimer, as I recall. Is that fixed
now?

[gab, 2017/06/22 16:03:59]

Yes base::Timer is now sequence-friendly

[Sigurður Ásgeirsson, 2017/06/22 18:47:28]

Acknowledged.

+GetDownloadTaskRunner();
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_DOWNLOAD_DOWNLOAD_TASK_RUNNER_H_