Place floats in layout 2020, but don't flow text around the floats yet.

This commit puts floats behind the `layout.floats.enabled` pref, because of the
following issues and unimplemented features:

* Inline formatting contexts don't take floats into account, so text doesn't
  flow around the floats yet.

* Non-floated block formatting contexts don't take floats into account, so BFCs
  can overlap floats.

* Block formatting contexts that contain floats don't expand vertically to
  contain all the floats. That is, floats can stick out the bottom of BFCs,
  contra spec.

components/layout_2020/display_list/mod.rs

@@ -200,6 +200,8 @@ impl Fragment {
                 Visibility::Hidden => (),
                 Visibility::Collapse => (),
             },
+            Fragment::HoistedFloat(_) => {},
+            Fragment::Float => {},
             Fragment::AbsoluteOrFixedPositioned(_) => {},
             Fragment::Anonymous(_) => {},
             Fragment::Image(i) => match i.style.get_inherited_box().visibility {