CSS-Typed-OM has the following hierarchy:
CSSStyleValue
- CSSNumericValue
- CSSUnitValue
- CSSMathValue
- ...various math functions...
Somewhat unintuitively, numbers are also stored using CSSUnitValue with
`unit = "number"`.
There are no distinct classes for LengthStyleValue, etc in the spec, but
they're convenient for us, so they are implemented as subclasses of
CSSUnitValue, at least for now.
(cherry picked from commit 895276426791567f22a612c4a40fa177571cc03e)
Having one StyleValue for `<number>` and `<integer>` is making user code
more complicated than it needs to be. We know based on the property
being parsed, whether it wants a `<number>` or an `<integer>`, so we
can use separate StyleValue types for these.