This will no longer be true once we implement at-rules nested inside
style rules, for example:
```css
.foo {
color: yellow;
@media (min-width: 800px) {
color: orange;
}
}
```
All its overrides return constants, and without virtual dispatch the
`qualified_layer_name` and `absolutized_selectors` functions can benefit
from slightly better optimizations.
`CSSRule`s aren't allocated that often, so the memory impact is minimal.