Prevent margin collapse in the root flow

According to the CSS specification, section 8.3.1, margins of the root
element's box do not collapse. Ensure that root flow margins do not
collapse during the assign heights phase.
Remove unnecessary curly braces
Fix a compilation issue

This change snuck in from a followup change.
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.
Rework the way that collapse-through offset is calculated

Parents of collapsed-through children should place their origin after
the collapsed margin-top of the child, but the height (and advance) of
the child as well as its effect on floats should be calculated by the
collapsed top and bottom margins of the child. When dealing with this
situation, return the collapsed margin-top and use it to calculate the
origin only.