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
electron::api::Ws::Type val) {
    using Type = electron::api::Ws::Type;
    std::string type;
    switch (val) {
      case Type::BACKGROUND_PAGE:
        type = "backgroundPage";
        break;
      case Type::BROWSER_WINDOW:
        type = "window";
        break;
      case Type::BROWSER_VIEW:
        type = "browserView";
        break;
      case Type::REMOTE:
        type = "remote";
        break;
      case Type::WEB_VIEW:
        type = "webview";
        break;
      case Type::OFF_SCREEN:
        type = "offscreen";
        break;
      default:
        break;
    }
    return gin::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate,