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
settings.SetBoolKey(printing::kSettingHeaderFooterEnabled, true);
    settings.SetStringKey(printing::kSettingHeaderFooterTitle, header);
    settings.SetStringKey(printing::kSettingHeaderFooterURL, footer);
  } else {
    settings.SetBoolKey(printing::kSettingHeaderFooterEnabled, false);
  }
  // We don't want to allow the user to enable these settings
  // but we need to set them or a CHECK is hit.
  settings.SetIntKey(printing::kSettingPrinterType, printing::kLocalPrinter);
  settings.SetBoolKey(printing::kSettingShouldPrintSelectionOnly, false);
  settings.SetBoolKey(printing::kSettingRasterizePdf, false);
  // Set custom page ranges to print
  std::vector<gin_helper::Dictionary> page_ranges;
  if (options.Get("pageRanges", &page_ranges)) {
    base::Value page_range_list(base::Value::Type::LIST);
    for (auto& range : page_ranges) {
      int from, to;
      if (range.Get("from", &from) && range.Get("to", &to)) {
        base::Value range(base::Value::Type::DICTIONARY);
        range.SetIntKey(printing::kSettingPageRangeFrom, from);
        range.SetIntKey(printing::kSettingPageRangeTo, to);
        page_range_list.Append(std::move(range));
      } else {
        continue;
      }
    }
    if (page_range_list.GetList().size() > 0)
      settings.SetPath(printing::kSettingPageRange, std::move(page_range_list));
  }
  // Duplex type user wants to use.