Implement a background downloader using BITS in Windows Chrome.

chrome/browser/component_updater/url_fetcher_downloader.h

 // Copyright 2013 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_COMPONENT_UPDATER_URL_FETCHER_DOWNLOADER_H_
 #define CHROME_BROWSER_COMPONENT_UPDATER_URL_FETCHER_DOWNLOADER_H_
 
 #include "chrome/browser/component_updater/crx_downloader.h"
 #include "net/url_request/url_fetcher_delegate.h"
 
 namespace net {
 class URLFetcher;
 }
 
 namespace component_updater {
 
 // Implements a CRX downloader in top of the URLFetcher.
 class UrlFetcherDownloader : public CrxDownloader,
                              public net::URLFetcherDelegate {
  protected:
   friend class CrxDownloader;
-  UrlFetcherDownloader(net::URLRequestContextGetter* context_getter,
-                       scoped_refptr<base::SequencedTaskRunner> task_runner);
+  UrlFetcherDownloader(scoped_ptr<CrxDownloader> successor,
+                       net::URLRequestContextGetter* context_getter,
+                       scoped_refptr<base::SequencedTaskRunner> task_runner,
+                       const DownloadCallback& download_callback);
   virtual ~UrlFetcherDownloader();
 
  private:
   // Overrides for CrxDownloader.
   virtual void DoStartDownload(const GURL& url) OVERRIDE;
 
   // Overrides for URLFetcherDelegate.
   virtual void OnURLFetchComplete(const net::URLFetcher* source) OVERRIDE;
 
   scoped_ptr<net::URLFetcher> url_fetcher_;
   net::URLRequestContextGetter* context_getter_;
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(UrlFetcherDownloader);
 };
 
 }  // namespace component_updater
 
 #endif  // CHROME_BROWSER_COMPONENT_UPDATER_URL_FETCHER_DOWNLOADER_H_