Some instances of CSSStyleProperties can lack an owner node, for
instance the return value of a call to `window.getComputedStyle` where
the specified pseudo-element is invalid. In this case we should treat
the computed style as empty, as there is no node to compute the style
for.
Previously, `CSSStyleSheet.replace()` and `CSSStyleSheet.replaceSync()`
parsed the given CSS text into a temporary stylesheet object, from
which a list of rules was extracted. Doing this had the unintended
side-effect that a fetch request would be started if the given CSS text
referenced any external resources. This fetch request would cause a
crash, since the temporary stylesheet object didn't set the constructed
flag, or constructor document. We now parse the given CSS text as a
list of rules without constructing a temporary stylesheet.
This improves the output of `getComputedStyle()` for the `top`,
`bottom`, `left` and `right` properties, where the used value is now
returned rather than the computed value, where applicable."
