This is unfortunate, but making that useful would require parameterizing
`SharedLayoutContext` and `LayoutContext` depending on the
`SelectorImpl` (which is a **huge** work right now).

Probably the easier way to do it, and probably the one that keeps the
layout code more legible, and since there won't be multiple
implementations at the same compilation unit, would be "defining" a
default implementation for layout via feature flags.

That should allow us to remove the components/style/servo.rs file.