Electron Version: 7.1.1
Operating System: Windows 10 (1803)

If I set the 'margins' option when call webContents.print method,
The app will crash with no error message.

```js
let { remote } = require("electron");
let webContents = remote.getCurrentWebContents();
webContents.print(
  {
    margins: {
      marginType: "custom",
      top: 30,
      bottom: 0,
      left: 0,
      right: 0
    }
  },
  (success, errorType) => {
    if (!success) console.log(errorType);
  }
);
```

shell/browser/api/electron_api_web_contents.cc
const printing::PrinterBasicInfo& val) {
    gin_helper::Dictionary dict = gin::Dictionary::CreateEmpty(isolate);
    dict.Set("name", val.printer_name);
    dict.Set("displayName", val.display_name);
    dict.Set("description", val.printer_description);
    dict.Set("status", val.printer_status);
    dict.Set("isDefault", val.is_default ? true : false);
    dict.Set("options", val.options);
    return dict.GetHandle();
  }
};
template <>
struct Converter<printing::MarginType> {
  static bool FromV8(v8::Isolate* isolate,