This exposed a few bugs which caused the following tests to behave
incorrectly:
- `tab-size-text-wrap.html`: This previously relied on a bug where we
incorrectly treated `white-space: pre` as allowing text wrapping. The
fix here is to implement the text-wrap CSS shorthand property.
- `execCommand-preserveWhitespace.html`: We don't correctly serialize
shorthand properties. This is covered by an existing FIXME in
`CSSStyleProperties::serialized()`
- `white-space-shorthand.html`: The last 5 subtests here fail as we
don't correctly handle shorthand properties in
`CSSStyleProperties::remove_property()`. This is covered by an
existing FIXME in said function.
This fixes one source of flakiness on WPT (of many) where we wouldn't
recompute style after programmatically altering the contents of a style
sheet, but instead had to wait for something else to cause invalidation.
The regression in the "conditional-CSSGroupingRule" test is we now fail
the "inserting an `@import`" subtests differently and the subtests
aren't independent. Specifically, we don't yet implement the checks in
`CSSRuleList::insert_a_css_rule()` that reject certain rules from being
inserted. Previously we didn't insert the `@import` rule because we
failed to parse its relative URL. Now we parse it correctly, we end up
inserting it.
