[css-values-4] Clarification on how non-calc() math functions serialize as specified values when nested in calc.

[weinig]

There is a WTP test, https://wpt.fyi/results/css/css-values/calc-complex-unresolved-serialize.html, that asserts that the specified value serialization of the following:

calc(pow(2, sign(1em - 18px)))

should maintain the outer calc, and be serialize as it is:

calc(pow(2, sign(1em - 18px)))

In WebKit, we currently remove the outer calc() and serialize it as pow(2, sign(1em - 18px)).

I can't figure out which is correct.

(cc @tabatkins)