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/lazy_instance.h"
+#include "base/task_scheduler/post_task.h"
+
+namespace content {
+
+namespace {
+
+struct DownloadTaskRunner {
+  const scoped_refptr<base::SingleThreadTaskRunner> task_runner =
+      base::CreateSingleThreadTaskRunnerWithTraits(
+          {base::MayBlock(), base::TaskPriority::BACKGROUND});

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

Isn't this USER_VISIBLE?

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

I'm not sure either way, the activity on this sequence is mostly about writing
files, though notifications go back to the user. Downloads aren't ever terribly
interactive though.

[gab, 2017/06/22 17:10:08]

Right not interactive but user visible, i.e. not USER_BLOCKING but USER_VISIBLE

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

Done.

+};
+
+base::LazyInstance<DownloadTaskRunner>::Leaky g_download_task_runner =
+    LAZY_INSTANCE_INITIALIZER;

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

Francois added a new API to do all of this in one line (instead of
struct+LazyInstance):

#include "base/task_scheduler/lazy_task_runner.h"

LazySingleThreadTaskRunner g_download_task_runner =
    LAZY_SINGLE_THREAD_TASK_RUNNER_INITIALIZER(
        base::TaskTraits(base::MayBlock(), 
                         base::TaskPriority::BACKGROUND),
    base::SingleThreadTaskRunnerThreadMode::SHARED);

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

Done.

+
+}  // namespace
+
+scoped_refptr<base::SingleThreadTaskRunner> GetDownloadTaskRunner() {
+  return g_download_task_runner.Get().task_runner;
+}
+
+}  // content