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
v8::Isolate* isolate,
      const atom::api::DesktopCapturer::Source& source) {
    mate::Dictionary dict(isolate, v8::Object::New(isolate));
    content::DesktopMediaID id = source.media_list_source.id;
    dict.Set("name", base::UTF16ToUTF8(source.media_list_source.name));
    dict.Set("id", id.ToString());
    dict.Set("thumbnail",
             atom::api::NativeImage::Create(
                 isolate, gfx::Image(source.media_list_source.thumbnail)));
    dict.Set("display_id", source.display_id);
    if (source.fetch_icon) {
      dict.Set(
          "appIcon",
          atom::api::NativeImage::Create(
              isolate, gfx::Image(GetWindowIcon(source.media_list_source.id))));
    }
    return ConvertToV8(isolate, dict);
  }
};
}  
namespace atom {
namespace api {
DesktopCapturer::DesktopCapturer(v8::Isolate* isolate) {