[css3-grid-layout] [css3-layout] fit-content and auto sizes

A minor grid-layout issue: 'auto' should not be a synonym for 'fit-content'.
It's a waste of a good keyword. While CSS3 has wound up introducing a few
aliases, it's something we have not had in CSS before and should be trying
to avoid.

That aside, elsewhere  'fit-content' is equivalent to
   max(min-content, min(fill-available, max-content))  // shrinkwrap formula
not to
   minmax(min-content, max-content)
Depending on any additional constraints in effect, though, this might wind
up meaning roughly the same thing. I haven't quite analyzed the algorithms
here... But for a grid element with a single track (or a template element
with a single slot), my expectation is that 'fit-content' would shrinkwrap
as defined above. Does it?

(If not, then we should use a different keyword here.)

~fantasai