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::RenderFrameHost* render_frame_host,
    const GURL& security_origin,
    blink::mojom::MediaStreamType type) {
  auto* ontents =
      content::Ws::FromRenderFrameHost(render_frame_host);
  auto* permission_helper =
      WsPermissionHelper::FromWs(ontents);
  return permission_helper->CheckMediaAccessPermission(security_origin, type);
}
void Ws::RequestMediaAccessPermission(
    content::Ws* ontents,
    const content::MediaStreamRequest& request,
    content::MediaResponseCallback callback) {
  auto* permission_helper =
      WsPermissionHelper::FromWs(ontents);
  permission_helper->RequestMediaAccessPermission(request, std::move(callback));
}
void Ws::RequestToLockMouse(content::Ws* ontents,
                                     bool user_gesture,
                                     bool last_unlocked_by_target) {
  auto* permission_helper =
      WsPermissionHelper::FromWs(ontents);
  permission_helper->RequestPointerLockPermission(user_gesture);
}
std::unique_ptr<content::BluetoothChooser> Ws::RunBluetoothChooser(
    content::RenderFrameHost* frame,
    const content::BluetoothChooser::EventHandler& event_handler) {
  return std::make_unique<BluetoothChooser>(this, event_handler);
}
content::JavaScriptDialogManager* Ws::GetJavaScriptDialogManager(