[css3-flexbox] negative flex-order

The old flex spec required natural numbers for box-ordinal-group and also
box-ordinal-group is 0 indexed instead of 1 indexed. This lets you avoid
doing an initial pass before you start laying out. You can layout in
document order until you hit a non-zero box-ordinal-group.

The new spec says flex-order is an "integer" and that it defaults to 1. We
should either require the flex-order be a positive integer or default it to
0 and require it to be non-negative.

Ojan