The exact same two steps are repeated for all of these in
CSSNumericValue. These steps are complicated enough that it makes
sense to factor them out into some helpers.
Because we store calculations as a tree of CalculationNodes inside a
CalculatedStyleValue, instead of a tree of StyleValues directly, this
implements a create_calculation_node() method on CSSNumericValue.
CSSMathValue::create_an_internal_representation() then calls
create_calculation_node() on itself, and wraps it in a
CalculatedStyleValue.
Lots of WPT passes again! Some regressions, which are expected: `cursor`
fails a test for the same reason it fails other that set it to some
kind of numeric value: We don't distinguish between "can contain a
number" and "can accept a number by itself". This will affect any
similar properties, but overall this is a big improvement.
