Proper static top of absolute elements in margin collapsed parents

When an absolutely positioned element has a static position in an
element with margins collapsed-through, the CSS spec says its y-position
should be defined by top margin (and not the collapsed top and bottom
margin) of the outermost collapsed-through ancestor. This patch ensures
that behavior.

A simple example of this behavior is that an absolutely positioned
element with "top: auto" should be placed after the top margin of the
body and not at 0, 0 on the page.