Reftests take screenshot too early in Chrome

[gsnedders]

Probably w3c/wptrunner#241

Almost identical to to #6328.

w3c/csswg-drafts#1088 is the issue that causes the issue here. Notably, wptrunner needs to block screenshots on fonts loading.

cc/ @jgraham @jugglinmike @bobholt

[gsnedders]

Actually, it's worse than that, as discussed in the email thread:

We need to take a screenshot after:

• after the document's load event has been fired
• the document.fonts.ready promise being fulfilled
• all @imports from stylesheets have been loaded
• after all subresources in all stylesheets have been loaded (so background-image, content, etc.)

And the last two of those can't be detected through JS. :\

[gsnedders]

OK, maybe the last two block the load event in Chrome per #5525 so maybe we're okay.

[foolip]

https://bugs.chromium.org/p/chromium/issues/detail?id=507054 is the Chromium bug tracking document.fonts.ready being fulfilled. That may not be the only required fix, but I think we should treat this as blocked on that.

[gsnedders]

@foolip that bug is only really about the Blink test runner, is it not? whereas, I believe, this is about wptrunner.