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 GURL& target_url,
    const content::Referrer& referrer,
    const std::string& frame_name,
    WindowOpenDisposition disposition,
    const std::vector<std::string>& features,
    const scoped_refptr<network::ResourceRequestBody>& body) {
  if (type_ == Type::BROWSER_WINDOW || type_ == Type::OFF_SCREEN)
    Emit("-new-window", target_url, frame_name, disposition, features, body,
         referrer);
  else
    Emit("new-window", target_url, frame_name, disposition, features);
}
void Ws::WsCreated(content::Ws* source_contents,
                                     int opener_render_process_id,
                                     int opener_render_frame_id,
                                     const std::string& frame_name,
                                     const GURL& target_url,
                                     content::Ws* new_contents) {
  ChildWsTracker::CreateForWs(new_contents);
  auto* tracker = ChildWsTracker::FromWs(new_contents);
  tracker->url = target_url;
  tracker->frame_name = frame_name;
}
void Ws::AddNewContents(