Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/download_task_runner.cc

+// 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/browser/download/download_task_runner.h"
+
+#include "base/task_scheduler/lazy_task_runner.h"
+
+namespace content {
+
+namespace {
+
+base::LazySequencedTaskRunner g_download_task_runner =
+    LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER(
+        base::TaskTraits(base::MayBlock(), base::TaskPriority::USER_VISIBLE));
+
+}  // namespace
+
+scoped_refptr<base::SequencedTaskRunner> GetDownloadTaskRunner() {
+  return g_download_task_runner.Get();
+}
+
+}  // namespace content