`EdgeStyleValues` which consist of an offset of a `calc()`s which
resolves to 50% should be considered "centered" for
`SerializationMode::ResolvedValue` for the purpose of omitting the
position value from gradient serialization.
As well as being required to implement absolutization this also means we
now bypass a limitation with `LengthPercentage` where we would always
use `SerializationMode::Normal` for the constituent lengths which gains
us some WPT passes
