LibWeb: Account for absolutely positioned table wrappers

[implicitfield]

Table wrappers don't quite behave the same as most elements, in that their computed height and width are not meant to be used for layout. Instead, we now calculate suitable widths and heights based on the contents of the table wrapper when performing absolute layout.

Fixes the layout of http://wpt.live/css/css-position/position-absolute-center-007.html