This doesn't need all of Selector.h and its various includes, it just
needs the PseudoElement enum.
StringStyleValue.h was transitively including ComponentValue.h through
this, so it now includes it directly.
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. 😅
A couple of arbitrary substitution functions require us to get or
produce some style value, and then substitute its ComponentValues into
the original ComponentValue list. So this commit gives CSSStyleValue a
tokenize() method that does so.
Apart from a couple of unusual cases like the guaranteed-invalid value,
style values can all be converted into ComponentValues by serializing
them as a string, and then parsing that as a list of component values.
That feels unnecessarily inefficient in most cases though, so I've
implemented faster overrides for a lot of the basic style value
classes, but left that serialize-and-reparse method as the fallback.
