[base] Hide loading screen when tab is not visible

[rexxars]

The new loading screen that fades in/out is pretty nice, but sometimes you are developing a feature without the browser open. Because the changes are live-reloaded, the loading screen will reappear on change, but it doesn't actually start fading out until you focus the tab.

This is a slight annoyance to me when the page is actually ready to go but not visible right away. Could we check document.visibilityState to see if the tab is hidden and if so instantly hide the loading screen?