Previously we were converting lengths to CSSPixels values when we didn't
need to, this had a couple of effects in that:
- We rounded to CSSPixel resolution prematurely (sometimes giving
incorrect results)
- We converted NaN to 0 when we shouldn't have.
We now avoid prematurely converting lengths to CSSPixels values in two
places:
- `CalculationResult::from_value`
- `CalculatedStyleValue::resolve_length_deprecated` (the new method
already avoided rounding).
Gains us 16 WPT tests.
