Add the download component and initial setup

chrome/browser/download/background_download_service_factory.h

+// 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 CHROME_BROWSER_DOWNLOAD_BACKGROUND_DOWNLOAD_SERVICE_FACTORY_H_
+#define CHROME_BROWSER_DOWNLOAD_BACKGROUND_DOWNLOAD_SERVICE_FACTORY_H_
+
+#include "base/macros.h"
+#include "components/keyed_service/content/browser_context_keyed_service_factory.h"
+
+namespace base {
+template <typename T>
+struct DefaultSingletonTraits;
+}  // namespace base
+
+namespace content {
+class BrowserContext;
+}  // namespace content
+
+namespace download {
+class BackgroundDownloadService;
+}  // namespace download
+
+// BackgroundDownloadServiceFactory is the main client class for interaction
+// with the downloads component.
+class BackgroundDownloadServiceFactory
+    : public BrowserContextKeyedServiceFactory {
+ public:
+  // Returns singleton instance of InternalDownloadServiceFactory.
+  static BackgroundDownloadServiceFactory* GetInstance();
+
+  // Returns the DownloadService associated with |context|.
+  static download::BackgroundDownloadService* GetForBrowserContext(
+      content::BrowserContext* context);
+
+ private:
+  friend struct base::DefaultSingletonTraits<BackgroundDownloadServiceFactory>;
+
+  BackgroundDownloadServiceFactory();
+  ~BackgroundDownloadServiceFactory() override;
+
+  // BrowserContextKeyedServiceFactory overrides:
+  KeyedService* BuildServiceInstanceFor(
+      content::BrowserContext* context) const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
+
+  DISALLOW_COPY_AND_ASSIGN(BackgroundDownloadServiceFactory);
+};
+
+#endif  // CHROME_BROWSER_DOWNLOAD_BACKGROUND_DOWNLOAD_SERVICE_FACTORY_H_