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
}
void Ws::SetAudioMuted(bool muted) {
  ontents()->SetAudioMuted(muted);
}
bool Ws::IsAudioMuted() {
  return ontents()->IsAudioMuted();
}
bool Ws::IsCurrentlyAudible() {
  return ontents()->IsCurrentlyAudible();
}
#if BUILDFLAG(ENABLE_PRINTING)
void Ws::OnGetDefaultPrinter(
    base::Value print_settings,
    printing::CompletionCallback print_callback,
    base::string16 device_name,
    bool silent,
    base::string16 default_printer) {
  // The content::Ws might be already deleted at this point, and the
  // PrintViewManagerBasic class does not do null check.
  if (!ontents()) {
    if (print_callback)
      std::move(print_callback).Run(false, "failed");
    return;
  }
  base::string16 printer_name =
      device_name.empty() ? default_printer : device_name;
  print_settings.SetStringKey(printing::kSettingDeviceName, printer_name);
  auto* print_view_manager =
      printing::PrintViewManagerBasic::FromWs(ontents());
  auto* focused_frame = ontents()->GetFocusedFrame();