- 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
#include "atom/browser/api/atom_api_download_item.h"
#include <map>
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/common/native_mate_converters/callback.h"
#include "atom/common/native_mate_converters/file_path_converter.h"
#include "atom/common/native_mate_converters/gurl_converter.h"
#include "base/message_loop/message_loop.h"
#include "base/strings/utf_string_conversions.h"
#include "native_mate/dictionary.h"
#include "net/base/filename_util.h"
#include "atom/common/node_includes.h"
namespace mate {
template<>
struct Converter<content::DownloadItem::DownloadState> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,