- 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
ClearStorageDataOptions options;
  base::Closure callback;
  args->GetNext(&options);
  args->GetNext(&callback);
  auto storage_partition =
      content::BrowserContext::GetStoragePartition(browser_context(), nullptr);
  storage_partition->ClearData(
      options.storage_types, options.quota_types, options.origin,
      content::StoragePartition::OriginMatcherFunction(),
      base::Time(), base::Time::Max(),
      base::Bind(&OnClearStorageDataDone, callback));
}
void Session::FlushStorageData() {
  auto storage_partition =
      content::BrowserContext::GetStoragePartition(browser_context(), nullptr);
  storage_partition->Flush();
}
void Session::SetProxy(const net::ProxyConfig& config,
                       const base::Closure& callback) {
  auto getter = browser_context_->GetRequestContext();
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&SetProxyInIO, base::Unretained(getter), config, callback));
}
void Session::SetDownloadPath(const base::FilePath& path) {