[test-runner-commands] Add Webdriver support for setViewportSize command

Consider using [`setWindowSize`](https://webdriver.io/docs/api/browser/setWindowSize.html) API provided by Webdriver to implement `setViewportSize` command:

```js
browser.setWindowSize(width, height);
```

This is needed in one of our visual regression tests so I'm going to handle this at some point 🙂 