This is intended as a replacement for Length and friends each holding a
`RefPtr<CalculatedStyleValue>`. Instead, let's make the types explicit
about whether they are calculated or not. This then means a Length is
always a Length, and won't require including `StyleValue.h`.
As noted, it's probably nicer for LengthOrCalculated to live in
`Length.h`, but we can't do that until Length stops including
`StyleValue.h`.
