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
return static_cast<ElectronBrowserContext*>(
      ontents()->GetBrowserContext());
}
// static
gin::Handle<Ws> Ws::Create(
    v8::Isolate* isolate,
    const gin_helper::Dictionary& options) {
  return gin::CreateHandle(isolate, new Ws(isolate, options));
}
// static
gin::Handle<Ws> Ws::CreateAndTake(
    v8::Isolate* isolate,
    std::unique_ptr<content::Ws> ontents,
    Type type) {
  return gin::CreateHandle(
      isolate, new Ws(isolate, std::move(ontents), type));
}
// static
gin::Handle<Ws> Ws::From(v8::Isolate* isolate,
                                           content::Ws* ontents) {
  auto* existing = TrackableObject::FromWrappedClass(isolate, ontents);
  if (existing)
    return gin::CreateHandle(isolate, static_cast<Ws*>(existing));
  else
    return gin::Handle<Ws>();
}
// static
gin::Handle<Ws> Ws::FromOrCreate(