Add the download component and initial setup

chrome/browser/download/download_service_factory.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 "chrome/browser/download/download_service_factory.h"
+
+#include "base/files/file_path.h"
+#include "base/sequenced_task_runner.h"
+#include "base/task_scheduler/post_task.h"
+#include "chrome/browser/profiles/incognito_helpers.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/common/chrome_constants.h"
+#include "components/download/public/download_service.h"
+#include "components/keyed_service/content/browser_context_dependency_manager.h"
+#include "content/public/browser/browser_context.h"
+
+// static
+DownloadServiceFactory* DownloadServiceFactory::GetInstance() {
+  return base::Singleton<DownloadServiceFactory>::get();
+}
+
+// static
+download::DownloadService* DownloadServiceFactory::GetForBrowserContext(
+    content::BrowserContext* context) {
+  return static_cast<download::DownloadService*>(
+      GetInstance()->GetServiceForBrowserContext(context, true));
+}
+
+DownloadServiceFactory::DownloadServiceFactory()
+    : BrowserContextKeyedServiceFactory(
+          "download::DownloadService",
+          BrowserContextDependencyManager::GetInstance()) {}
+
+DownloadServiceFactory::~DownloadServiceFactory() = default;
+
+KeyedService* DownloadServiceFactory::BuildServiceInstanceFor(
+    content::BrowserContext* context) const {
+  Profile* profile = Profile::FromBrowserContext(context);
+
+  scoped_refptr<base::SequencedTaskRunner> background_task_runner =
+      base::CreateSequencedTaskRunnerWithTraits(
+          base::TaskTraits().MayBlock().WithPriority(
+              base::TaskPriority::BACKGROUND));

[dcheng, 2017/05/04 00:19:26]

Based on CLs like https://codereview.chromium.org/2858073002/, I think this
should use the constexpr overload if possible: for example, I think this can be
written

base::CreateSequencedTaskRunnerWithTraits({base::MayBlock(),
base::TaskPriority::BACKGROUND})

[David Trainor- moved to gerrit, 2017/05/04 16:15:48]

Done.

+
+  base::FilePath storage_dir;
+  if (!profile->IsOffTheRecord() && !profile->GetPath().empty()) {
+    storage_dir =
+        profile->GetPath().Append(chrome::kDownloadServiceStorageDirname);
+  }
+
+  download::DownloadService* service =
+      download::DownloadService::Create(storage_dir, background_task_runner);
+
+  // TODO(dtrainor): Register all clients here.
+  return service;
+}
+
+content::BrowserContext* DownloadServiceFactory::GetBrowserContextToUse(
+    content::BrowserContext* context) const {
+  return chrome::GetBrowserContextOwnInstanceInIncognito(context);
+}