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
content::Ws* source,
    std::unique_ptr<content::Ws> new_contents,
    WindowOpenDisposition disposition,
    const gfx::Rect& initial_rect,
    bool user_gesture,
    bool* was_blocked) {
  auto* tracker = ChildWsTracker::FromWs(new_contents.get());
  DCHECK(tracker);
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  auto api_ontents =
      CreateAndTake(isolate(), std::move(new_contents), Type::BROWSER_WINDOW);
  if (Emit("-add-new-contents", api_ontents, disposition, user_gesture,
           initial_rect.x(), initial_rect.y(), initial_rect.width(),
           initial_rect.height(), tracker->url, tracker->frame_name)) {
    
    api_ontents->DestroyWs(true );
  }
}
content::Ws* Ws::OpenURLFromTab(