LibWeb: Correctly calculate height of TableRowGroupBox

As well as correctly calculating the height of TableRowBox, this change
calculates the heights of TableRowGroupBoxs also.

As before, this does not correctly take into consideration the 'height'
attribute.

Now the horizontal layout is approximately correct for the
TableRowGroupBoxs we can now see that the `layout_row` method will need
updating to correctly calculate cell width across all rows, not just the
current TableRowGroupBox.

Userland/Libraries/LibWeb/Layout/TableFormattingContext.cpp

@@ -71,6 +71,7 @@ void TableFormattingContext::run(Box& box, LayoutMode)
 
  row_group_box.set_height(content_height);
 
+ row_group_box.set_offset(0, total_content_height);
  total_content_height += content_height;
  });