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
}
Ws::Ws(v8::Isolate* isolate,
                         std::unique_ptr<content::Ws> ontents,
                         Type type)
    : content::WsObserver(ontents.get()),
      type_(type),
      weak_factory_(this) {
  DCHECK(type != Type::REMOTE)
      << "Can't take ownership of a remote Ws";
  auto session = Session::CreateFrom(isolate, GetBrowserContext());
  session_.Reset(isolate, session.ToV8());
  InitWithSessionAndOptions(isolate, std::move(ontents), session,
                            gin::Dictionary::CreateEmpty(isolate));
}
Ws::Ws(v8::Isolate* isolate,
                         const gin_helper::Dictionary& options)
    : weak_factory_(this) {
  
  options.Get("backgroundThrottling", &background_throttling_);
  
  options.Get("type", &type_);
#if BUILDFLAG(ENABLE_OSR)
  bool b = false;
  if (options.Get(options::kOffscreen, &b) && b)
    type_ = Type::OFF_SCREEN;
#endif
  
  options.Get("embedder", &embedder_);
  
  options.Get("devTools", &enable_devtools_);