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.
We replace Au with CSSPixelLength for the length part of
computed::CalcLengthOrPercentage. Therefore, it would be easier to use
CSSPixelLength for all other LengthOrPercentage{*} types.
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>)
As silly as it may seem to specify font-sizes using viewport units, we weren't
handling zoom for them correctly either.
Bug: 1388588
Reviewed-by: Manishearth
MozReview-Commit-ID: 3Q6phYAu5CE
