- Electron version: 1.4.3
- Operating system:  @win7
- description:

working on a download app with electron and angular. 
By using webContents.downloadUrl, webContents.session.on('will-download', handler) I can make download function work well, however, I could not find a api to save download progress before closing electron app so that I can restore download when this app is started next time.

Excepted Result:
1. user click 'download' button to download a 1G file.
2. after 500M is downloaded, user quit the app
3. one hour later, user open this app again, and downloading can automatically be restored

atom/browser/api/atom_api_download_item.cc
content::DownloadItem::DownloadState state) {
    std::string download_state;
    switch (state) {
      case content::DownloadItem::IN_PROGRESS:
        download_state = "progressing";
        break;
      case content::DownloadItem::COMPLETE:
        download_state = "completed";
        break;
      case content::DownloadItem::CANCELLED:
        download_state = "cancelled";
        break;
      case content::DownloadItem::INTERRUPTED:
        download_state = "interrupted";
        break;
      default:
        break;
    }
    return ConvertToV8(isolate, download_state);
  }
};
}  
namespace atom {
namespace api {
namespace {
std::map<uint32_t, v8::Global<v8::Object>> g_download_item_objects;
}  
DownloadItem::DownloadItem(v8::Isolate* isolate,