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.
Shorthand subproperties that match their initial values are now
excluded from serialization, by default.
Properties where this behavior is not desired, like `gap`, are
special-cased.
This implementation also fixes an issue where the individual components
of the `border-radius` shorthand were always assumed to be of type
`BorderRadiusStyleValue`, which could lead to a crash when CSS-wide
keywords were used.
Regardless of what the shorthand property is, if all its longhands are
the same CSS-wide keyword such as "initial" or "inherit", then it's the
same as the shorthand being that value.
This gets us 2 WPT subtest passes.
