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
#if defined(OS_MACOSX)
  base::ScopedCFTypeRef<CFStringRef> new_printer_id(
      base::SysUTF16ToCFStringRef(device_name));
  PMPrinter new_printer = PMPrinterCreateFromPrinterID(new_printer_id.get());
  bool printer_exists = new_printer != nullptr;
  PMRelease(new_printer);
  return printer_exists;
#elif defined(OS_WIN)
  printing::ScopedPrinterHandle printer;
  return printer.OpenPrinterWithName(device_name.c_str());
#endif
  return true;
}
base::string16 GetDefaultPrinterAsync() {