Some dimensions would always serialize in a canonical unit, others never
did, and others we manually would do so in their StyleValue. This
commit moves all of that into the dimension types, which means for
example that Length can apply its special rounding.
Our local serialization test now produces the same output as other
browsers. :^)
Previously, when serializing a time value, we would always convert it
to seconds. We now canonicalize the time value only when serializing
its computed value.
