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
Init(isolate);
}
DesktopCapturer::~DesktopCapturer() {}
void DesktopCapturer::StartHandling(bool capture_window,
                                    bool capture_screen,
                                    const gfx::Size& thumbnail_size,
                                    bool fetch_window_icons) {
  fetch_window_icons_ = fetch_window_icons;
#if defined(OS_WIN)
  if (content::desktop_capture::CreateDesktopCaptureOptions()
          .allow_directx_capturer()) {
    
    
    auto duplicator = webrtc::DxgiDuplicatorController::Instance();
    using_directx_capturer_ = webrtc::ScreenCapturerWinDirectx::IsSupported();
  }
#endif  
  
  captured_sources_.clear();
  
  capture_window_ = capture_window;
  capture_screen_ = capture_screen;
  {