I was looking today for a quick way to preview my game in chrome, in different aspect ratios and could not find a way, so I wrote a Google Chrome extension which lets me do so.
If you think it might be useful you can get it here,
h t t p s ://chrome.google.com/webstore/detail/aspect-ratio/bbelgfcfpiocfblomapnagpaagjfgkom
(could not enter proper link, since I am new here and apprently I do not have permission to do so yet)