browser_scroll_size_determination.js is occasionally leaking browser windows. My theory is that this is happening because `BrowserTestUtils.overflowTabs` keeps them alive in order to reset the scrollbox attribute that it changed and then there is not enough time after cleanup for the windows to get fully collected. Given that these windows are closed anyway there is no need to actually do this cleanup step so this allows us to skip it. Differential Revision: https://phabricator.services.mozilla.com/D229587