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
v8::Local<v8::Value> val,
                     printing::DuplexMode* out) {
    std::string mode;
    if (ConvertFromV8(isolate, val, &mode)) {
      if (mode == "simplex") {
        *out = printing::SIMPLEX;
        return true;
      }
      if (mode == "longEdge") {
        *out = printing::LONG_EDGE;
        return true;
      }
      if (mode == "shortEdge") {
        *out = printing::SHORT_EDGE;
        return true;
      }
    }
    return false;
  }
};
#endif
template <>
struct Converter<WindowOpenDisposition> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,