- 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
GURL pac_url;
    mate::Dictionary options;
    
    
    if (ConvertFromV8(isolate, val, &proxy_rules)) {
      pac_url = GURL(proxy_rules);  
    } else if (ConvertFromV8(isolate, val, &options)) {
      options.Get("pacScript", &pac_url);
      options.Get("proxyRules", &proxy_rules);
      options.Get("proxyBypassRules", &proxy_bypass_rules);
    } else {
      return false;
    }
    
    if (!pac_url.is_empty() && pac_url.is_valid()) {
      out->set_pac_url(pac_url);
    } else {
      out->proxy_rules().ParseFromString(proxy_rules);
      out->proxy_rules().bypass_rules.ParseFromString(proxy_bypass_rules);
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
const char kPersistPrefix[] = "persist:";
std::map<uint32_t, v8::Global<v8::Object>> g_sessions;
class ResolveProxyHelper {