This reverts 0e3487b9ab.
Back when I made that change, I thought we could make our StyleValue
classes match the typed-om definitions directly. However, they have
different requirements. Typed-om types need to be mutable and GCed,
whereas StyleValues are immutable and ideally wouldn't require a JS VM.
While I was already making such a cataclysmic change, I've moved it into
the StyleValues directory, because it *not* being there has bothered me
for a long time. 😅
The CSS `fetch_foo()` functions resolve the URL relative to the
CSSStyleSheet if one is provided. So, style values that do so need to
know what CSSStyleSheet they are part of so that, for example, `url
(foo.png)` is loaded relative to the style sheet's URL instead of the
document's one.
That all works without this change because we currently absolutize URLs
during parsing, but we're in the process of stopping that.
This commit adds the infrastructure for telling style values what their
CSSStyleSheet is.
