This more concrete wrapper type can write a prefix the very first time something
is written to it. This allows removing plenty of useless monomorphisations caused
by the former W/SequenceWriter<W> pair of types.
It is bogus, because it depends on the display property as it's cascaded, but
the display property can change afterwards, for example, if we get blockified
because we're the root element or a flex item.
Replace it with a normal field instead.
Also, it carries some weight, because it's the last property that uses this
concept of "derived" property, and "custom cascade". So we can remove some code
after this.
Compute it after the cascade process in StyleAdjuster.
First, we define computed::CSSPixelLength which contains a CSSFloat, a
pixel value, and then we replace computed::Length with CSSPixelLength.
Therefore, the |ComputedValue| of NoCalcLength, AbsoluteLength,
FontRelativeLength, ViewportPercentageLength, CharacterWidth, and
PhysicalLength is CSSPixelLength.
Besides, we drop NonNegativeAu, and replace computed::NonNegativeLength
with NonNegative<computed::Length>. (i.e. NonNegative<CSSPixelLength>)
