[css3-flexbox] forced breaks on flexbox items in multi-line rows

Right now the spec says:
   # but with the item causing the break automatically starting a new line in the step

Firstly, this makes no sense for break-after.

Secondly, this means that an item can cause a flex line break in paged mode but not
in continuous mode. I don't think this is a good idea: it should either always cause
a line break, or never cause a line break. Therefore IMO forced breaks on items in
row flexboxes should be propagated to the flex line and ignored on the item itself.

~fantasai