Race with setting tab metadata

Culprit is `handleOnload`: https://github.com/MercuryWorkshop/aboutproxy/blob/a7ace34f9f48787d703746a88f2d1f365b68de3c/static/browser.js#L292 Repro: - go to a location in one tab - before tab has fully loaded, change tabs - tab metadata will get plastered on the new tab you just opened