style: Fix one minor serialization issue which was causing test failures.
style: Fix one minor serialization issue which was causing test failures.

We were serializing calc(10% + 4px) as calc(10% + calc(4px)).

Differential Revision: https://phabricator.services.mozilla.com/D63400

KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo