- 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
v8::Local<v8::Value> val,
                     ClearStorageDataOptions* out) {
    mate::Dictionary options;
    if (!ConvertFromV8(isolate, val, &options))
      return false;
    options.Get("origin", &out->origin);
    std::vector<std::string> types;
    if (options.Get("storages", &types))
      out->storage_types = GetStorageMask(types);
    if (options.Get("quotas", &types))
      out->quota_types = GetQuotaMask(types);
    return true;
  }
};
template<>
struct Converter<net::ProxyConfig> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Local<v8::Value> val,
                     net::ProxyConfig* out) {
    std::string proxy_rules, proxy_bypass_rules;