electron code head commit:  2fb9085e5b9b28b7adf0ab7de720c87461b6689d
os: win10 64bit
visual studio 2019 preview

after build electron , I run 
```
electron.exe electron/spec
```
and the program crash, i use vs2019 debug it, see the picture, 
![捕获](https://user-images.githubusercontent.com/43543863/54986357-7331d580-4fed-11e9-9419-0775d533d3fe.PNG)

device_names is empty, and the code does not check it
atom/browser/api/atom_api_desktop_capturer.cc
#include "atom/browser/api/atom_api_desktop_capturer.h"
#include <memory>
#include <utility>
#include <vector>
#include "atom/common/api/atom_api_native_image.h"
#include "atom/common/native_mate_converters/gfx_converter.h"
#include "atom/common/node_includes.h"
#include "base/strings/string_number_conversions.h"
#include "base/strings/utf_string_conversions.h"
#include "base/threading/thread_restrictions.h"
#include "chrome/browser/media/webrtc/desktop_media_list.h"
#include "chrome/browser/media/webrtc/window_icon_util.h"
#include "content/public/browser/desktop_capture.h"
#include "native_mate/dictionary.h"
#include "third_party/webrtc/modules/desktop_capture/desktop_capture_options.h"
#include "third_party/webrtc/modules/desktop_capture/desktop_capturer.h"
#if defined(OS_WIN)
#include "third_party/webrtc/modules/desktop_capture/win/dxgi_duplicator_controller.h"
#include "third_party/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h"
#include "ui/display/win/display_info.h"
#endif  
namespace mate {
template <>
struct Converter<atom::api::DesktopCapturer::Source> {
  static v8::Local<v8::Value> ToV8(