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
if (IsOffScreen() && wc == ontents()) {
    auto* relay = NativeWindowRelay::FromWs(ontents());
    if (relay) {
      auto* owner_window = relay->GetNativeWindow();
      return owner_window ? owner_window->GetSize() : gfx::Size();
    }
  }
  return gfx::Size();
}
void Ws::SetZoomLevel(double level) {
  zoom_controller_->SetZoomLevel(level);
}
double Ws::GetZoomLevel() const {
  return zoom_controller_->GetZoomLevel();
}
void Ws::SetZoomFactor(double factor) {
  auto level = blink::PageZoomFactorToZoomLevel(factor);
  SetZoomLevel(level);
}
double Ws::GetZoomFactor() const {
  auto level = GetZoomLevel();
  return blink::PageZoomLevelToZoomFactor(level);
}
void Ws::SetTemporaryZoomLevel(double level) {
  zoom_controller_->SetTemporaryZoomLevel(level);
}
void Ws::DoGetZoomLevel(DoGetZoomLevelCallback callback) {
  std::move(callback).Run(GetZoomLevel());
}
std::vector<base::FilePath::StringType> Ws::GetPreloadPaths() const {