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
WindowOpenDisposition val) {
    std::string disposition = "other";
    switch (val) {
      case WindowOpenDisposition::CURRENT_TAB:
        disposition = "default";
        break;
      case WindowOpenDisposition::NEW_FOREGROUND_TAB:
        disposition = "foreground-tab";
        break;
      case WindowOpenDisposition::NEW_BACKGROUND_TAB:
        disposition = "background-tab";
        break;
      case WindowOpenDisposition::NEW_POPUP:
      case WindowOpenDisposition::NEW_WINDOW:
        disposition = "new-window";
        break;
      case WindowOpenDisposition::SAVE_TO_DISK:
        disposition = "save-to-disk";
        break;
      default:
        break;
    }
    return gin::ConvertToV8(isolate, disposition);
  }
};
template <>
struct Converter<content::SavePageType> {
  static bool FromV8(v8::Isolate* isolate,