Any type of Size which has no LengthPercentage value now uses an empty
optional instead of making an auto Length as before.
We also now serialize a `fit-content` Size as `fit-content` instead of
`fit-content(auto)`, though this doesn't affect test results and I
didn't identify where it's actually used.
Before this change, we only parsed fit-content as a standalone keyword,
but CSS-SIZING-3 added it as a function as well. I don't know of
anything else in CSS that is overloaded like this, so it ends up looking
a little awkward in the implementation.
Note that a lot of code had already been prepped for fit-content values
to have an argument, we just weren't parsing it.
This reverts commit 76daba3069.
We're going to need separate types for the JS-exposed style values, so
it doesn't make sense for us to match their names with our internal
types.
