Add the download component and initial setup

chrome/browser/download/download_service_factory.h

Index: chrome/browser/download/download_service_factory.h
diff --git a/chrome/browser/download/download_service_factory.h b/chrome/browser/download/download_service_factory.h
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+++ b/chrome/browser/download/download_service_factory.h
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+// 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_DOWNLOAD_SERVICE_FACTORY_H_
+#define CHROME_BROWSER_DOWNLOAD_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 DownloadService;
+}  // namespace download
+
+// DownloadServiceFactory is the main client class for interaction with the
+// download component.
+class DownloadServiceFactory : public BrowserContextKeyedServiceFactory {
+ public:
+  // Returns singleton instance of DownloadServiceFactory.
+  static DownloadServiceFactory* GetInstance();
+
+  // Returns the DownloadService associated with |context|.
+  static download::DownloadService* GetForBrowserContext(
+      content::BrowserContext* context);
+
+ private:
+  friend struct base::DefaultSingletonTraits<DownloadServiceFactory>;
+
+  DownloadServiceFactory();
+  ~DownloadServiceFactory() override;
+
+  // BrowserContextKeyedServiceFactory overrides:
+  KeyedService* BuildServiceInstanceFor(
+      content::BrowserContext* context) const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
+
+  DISALLOW_COPY_AND_ASSIGN(DownloadServiceFactory);
+};
+
+#endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_SERVICE_FACTORY_H_