[css-flexbox] Specify that auto cross-size of flexbox containers is still clamped by min/max-size.
[css-flexbox] Also apply the clamping to the first clause.
[css-flexbox] Update issues list
[css-flexbox] Add to changes.

--HG--
extra : rebase_source : cad00b9a567f7c88834c9701ef0d3b0150795566