Previously, `<link rel=stylesheet>` would delay the load event until its
style sheet loaded, but not care about its subresources. `<style>`
would not delay the load event at all. Instead, each `@import` would
delay the load event.
Now, both `<style>` and `<link>` delay the load event until their style
sheet and its critical subresources have loaded or failed. This means
that CSSImportRules no longer need to delay the load event themselves,
because they do so implicitly as a critical subresource of their parent
style sheet.
This doesn't directly affect behavior, but means that any other critical
style resources we add will automatically delay the load event.
One wrinkle here is that the spec for the `<link>` element requires that
we wait for the style sheet's critical subresources *before* we create
a CSSStyleSheet, which means we don't yet know what those are.
https://html.spec.whatwg.org/multipage/semantics.html#fetching-and-processing-a-resource-from-a-link-element:critical-subresources
For now we simply ignore this, as we did before. That means we continue
to not delay the `<link>`'s load event.
Previously, we fired the load event immediately, without waiting for
anything. This was good for not timing out, but bad for anything that
wanted to wait for the load to complete.
CSSStyleSheet now maintains a list of critical subresources, and waits
for all of them to complete before it then tells its owner that it is
ready. "Complete" here means the network request completed with or
without an error. This is done by having those subresources (just
`@import` for now) notify their style sheet when they complete. This
then propagates up as an `@import` tells its style sheet, which then
would tell its parent `@import` if it had one.
There are other subresources we should wait for (specifically fonts and
background images) but this commit just adds `@import` as a first step.
