Don't download extensions updates when background networking is disabled

When the "--disable-background-networking" command-line switch is
specified, extensions updates shouldn't be downloaded.

BUG=489244

Review URL: https://codereview.chromium.org/1140873004

Cr-Commit-Position: refs/heads/master@{#330388}

extensions/browser/updater/extension_downloader.cc

@@ -440,6 +440,7 @@ void ExtensionDownloader::StartUpdateCheck(
     NotifyExtensionsDownloadFailed(id_set,
                                    fetch_data->request_ids(),
                                    ExtensionDownloaderDelegate::DISABLED);
+    return;
   }
 
   RequestQueue<ManifestFetchData>::iterator i;