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::kSettingLandscape, landscape);
  // We set the default to the system's default printer and only update
  // if at the Chromium level if the user overrides.
  // Printer device name as opened by the OS.
  base::string16 device_name;
  options.Get("deviceName", &device_name);
  if (!device_name.empty() && !IsDeviceNameValid(device_name)) {
    args->ThrowError("ws.print(): Invalid deviceName provided.");
    return;
  }
  int scale_factor = 100;
  options.Get("scaleFactor", &scale_factor);
  settings.SetIntKey(printing::kSettingScaleFactor, scale_factor);
  int pages_per_sheet = 1;
  options.Get("pagesPerSheet", &pages_per_sheet);
  settings.SetIntKey(printing::kSettingPagesPerSheet, pages_per_sheet);
  // True if the user wants to print with collate.
  bool collate = true;
  options.Get("collate", &collate);
  settings.SetBoolKey(printing::kSettingCollate, collate);
  // The number of individual copies to print
  int copies = 1;
  options.Get("copies", &copies);
  settings.SetIntKey(printing::kSettingCopies, copies);
  // Strings to be printed as headers and footers if requested by the user.
  std::string header;
  options.Get("header", &header);
  std::string footer;
  options.Get("footer", &footer);
  if (!(header.empty() && footer.empty())) {