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

[web-padawan]

Consider using setWindowSize API provided by Webdriver to implement setViewportSize command:

browser.setWindowSize(width, height);

This is needed in one of our visual regression tests so I'm going to handle this at some point 🙂

[web-padawan]

Looks like there is a new setViewport() API in webdriver v9 which might be a better fit for this.