- 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_session.cc
content::DownloadItem* item) {
  if (item->IsSavePackageDownload())
    return;
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit(
      "will-download",
      DownloadItem::Create(isolate(), item),
      item->GetWebContents());
  if (prevent_default) {
    item->Cancel(true);
    item->Remove();
  }
}
void Session::ResolveProxy(const GURL& url, ResolveProxyCallback callback) {
  new ResolveProxyHelper(browser_context(), url, callback);
}
template<Session::CacheAction action>
void Session::DoCacheAction(const net::CompletionCallback& callback) {
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&DoCacheActionInIO,
                 make_scoped_refptr(browser_context_->GetRequestContext()),
                 action,
                 callback));
}
void Session::ClearStorageData(mate::Arguments* args) {