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
const gin_helper::Dictionary& options) {
  WsZoomController::CreateForWs(ontents);
  zoom_controller_ = WsZoomController::FromWs(ontents);
  double zoom_factor;
  if (options.Get(options::kZoomFactor, &zoom_factor))
    zoom_controller_->SetDefaultZoomFactor(zoom_factor);
}
void Ws::InitWithSessionAndOptions(