- 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
#include "base/threading/thread_task_runner_handle.h"
#include "brightray/browser/net/devtools_network_conditions.h"
#include "brightray/browser/net/devtools_network_controller_handle.h"
#include "chrome/common/pref_names.h"
#include "components/prefs/pref_service.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/storage_partition.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "net/base/load_flags.h"
#include "net/disk_cache/disk_cache.h"
#include "net/dns/host_cache.h"
#include "net/http/http_auth_handler_factory.h"
#include "net/http/http_auth_preferences.h"
#include "net/proxy/proxy_config_service_fixed.h"
#include "net/proxy/proxy_service.h"
#include "net/url_request/static_http_user_agent_settings.h"
#include "net/url_request/url_request_context.h"
#include "net/url_request/url_request_context_getter.h"
#include "ui/base/l10n/l10n_util.h"
using atom::api::Cookies;
using content::BrowserThread;
using content::StoragePartition;
namespace {
struct ClearStorageDataOptions {
  GURL origin;
  uint32_t storage_types = StoragePartition::REMOVE_DATA_MASK_ALL;
  uint32_t quota_types = StoragePartition::QUOTA_MANAGED_STORAGE_MASK_ALL;
};
uint32_t GetStorageMask(const std::vector<std::string>& storage_types) {