LibWeb: Fix wrong height:auto computation for block with floating child

If an element with height:auto has any floating descendants whose bottom
margin edge is below the element's bottom content edge, then the height
is increased to include those edges.

Before this patch, we were stopping at the bottom *content* edge of
floating descendants.

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -260,7 +260,7 @@ float FormattingContext::compute_auto_height_for_block_level_element(FormattingS
 
  auto const& child_box_state = state.get(child_box);
 
- float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height;
+ float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height + child_box_state.margin_box_bottom();
 
  if (!bottom.has_value() || child_box_bottom > bottom.value())
  bottom = child_box_bottom;