RFC 213: Wait for document.fonts.ready before running checkLayout() tests

[jonathan-j-lee]

Rendered

Tracking issue: https://crbug.com/375344326

[gsnedders]

I wonder, without thinking about this much, whether we should use the same logic as reftests (and crashtests, incidentally) use: https://github.com/web-platform-tests/wpt/blob/6c7c4003dba847b63e16083a0b7b8f473d9ad7e1/tools/wptrunner/wptrunner/executors/test-wait.js

Notably, this waits for the load event (and in some browsers this might wait for font loading, c.f. w3c/csswg-drafts#1088), which maybe relevant for rendering when images for example are involved, and then waits for document.fonts.ready and then two further rAFs (which we probably don't need).