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 {

components/layout_2020/display_list/stacking_context.rs

@@ -7,7 +7,7 @@ use crate::cell::ArcRefCell;
 use crate::display_list::conversions::ToWebRender;
 use crate::display_list::DisplayListBuilder;
 use crate::fragment_tree::ContainingBlockManager;
-use crate::fragments::{AnonymousFragment, BoxFragment, Fragment};
+use crate::fragments::{AnonymousFragment, BoxFragment, Fragment, HoistedFloatFragment};
 use crate::geom::PhysicalRect;
 use crate::style_ext::ComputedValuesExt;
 use crate::FragmentTree;
@@ -554,6 +554,14 @@ impl Fragment {
                     stacking_context,
                 );
             },
+            Fragment::HoistedFloat(fragment) => {
+                fragment.build_stacking_context_tree(
+                    display_list,
+                    containing_block_info,
+                    stacking_context,
+                );
+            },
+            Fragment::Float => {},
             Fragment::AbsoluteOrFixedPositioned(fragment) => {
                 let shared_fragment = fragment.borrow();
                 let fragment_ref = match shared_fragment.fragment.as_ref() {
@@ -1095,3 +1103,20 @@ impl AnonymousFragment {
         }
     }
 }
+
+impl HoistedFloatFragment {
+    fn build_stacking_context_tree(
+        &self,
+        display_list: &mut DisplayList,
+        containing_block_info: &ContainingBlockInfo,
+        stacking_context: &mut StackingContext,
+    ) {
+        self.fragment.borrow().build_stacking_context_tree(
+            &self.fragment,
+            display_list,
+            containing_block_info,
+            stacking_context,
+            StackingContextBuildMode::SkipHoisted,
+        );
+    }
+}