document.fonts.ready() does not wait for fonts loading

[kalenikaliaksandr]

This API is used by WPT to wait until Ahem (testing font) is loaded before capturing layout measurement. Since currently our implementation of ready() simple returns resolved promise right away, instead of actually waiting for fonts loading, a giant portion of WPT layout test are flaky.

Moreover document.fonts currently does not include fonts defined in CSS using @font-face, so it might be a good idea to solve that first.

Related spec https://drafts.csswg.org/css-font-loading/