'min-height' can prevent margin collapsing through an element, like 'height'
auto merge of #2038 : SimonSapin/servo/min-height-collapse, r=pcwalton

[Relevant spec](http://dev.w3.org/csswg/css2/box.html#collapsing-margins):

> top and bottom margins of a box that does not establish a new block formatting context and that has zero computed 'min-height', zero or 'auto' computed 'height', and no in-flow children