Re: [css3-flexbox] Baseline of flexboxes and flexbox items

The baseline of a flexbox item is just whatever their display type
says it should be (there's no "flexbox item" display type).

The baseline of a flexbox, though, is indeed undefined.  Hmm.  Let me
see what the test implementations currently do.

(Unfortunately, we didn't get to discuss the issue at the last telcon
- ran out of time.)

~TJ