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::DownloadURL(const GURL& url) {
  auto* browser_context = ontents()->GetBrowserContext();
  auto* download_manager =
      content::BrowserContext::GetDownloadManager(browser_context);
  std::unique_ptr<download::DownloadUrlParameters> download_params(
      content::DownloadRequestUtils::CreateDownloadForWsMainFrame(
          ontents(), url, MISSING_TRAFFIC_ANNOTATION));
  download_manager->DownloadUrl(std::move(download_params));
}
GURL Ws::GetURL() const {
  return ontents()->GetURL();
}
base::string16 Ws::GetTitle() const {
  return ontents()->GetTitle();
}
bool Ws::IsLoading() const {
  return ontents()->IsLoading();
}
bool Ws::IsLoadingMainFrame() const {
  return ontents()->IsLoadingToDifferentDocument();
}
bool Ws::IsWaitingForResponse() const {
  return ontents()->IsWaitingForResponse();
}
void Ws::Stop() {
  ontents()->Stop();
}
void Ws::GoBack() {
  electron::ElectronBrowserClient::SuppressRendererProcessRestartForOnce();
  ontents()->GetController().GoBack();