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::MarginType* out) {
    std::string type;
    if (ConvertFromV8(isolate, val, &type)) {
      if (type == "default") {
        *out = printing::DEFAULT_MARGINS;
        return true;
      }
      if (type == "none") {
        *out = printing::NO_MARGINS;
        return true;
      }
      if (type == "printableArea") {
        *out = printing::PRINTABLE_AREA_MARGINS;
        return true;
      }
      if (type == "custom") {
        *out = printing::CUSTOM_MARGINS;
        return true;
      }
    }
    return false;
  }
};
template <>
struct Converter<printing::DuplexMode> {
  static bool FromV8(v8::Isolate* isolate,