see chrome/browser/chromeos/customization/customization_wallpaper_downloader.cc

 84 void CustomizationWallpaperDownloader::StartRequest() {
 85   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 86   DCHECK(wallpaper_url_.is_valid());
 87 
 (...)
 94   // TODO(crbug.com/): Add a real traffic annotation here.
 95   simple_loader_ = network::SimpleURLLoader::Create(std::move(resource_request),
 96                                                     MISSING_TRAFFIC_ANNOTATION);