Don't recalc assignement in HTMLSlotElement::DetachLayoutTree

The workaround of https://crrev.com/c/1062990 is no longer necessary
because of https://crrev.com/c/1068646, which fixed ListItemOrdinal's behavior.

See also http://crbug.com/845770. In general, we should avoid recalc assignment
in detaching a node.

Bug: 776656, 845770
Change-Id: Ia98b35dce0b6d2e16829f79dde4b88470ced0d3f
Reviewed-on: https://chromium-review.googlesource.com/1070007Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561371}

third_party/blink/renderer/core/html/html_slot_element.cc

@@ -364,16 +364,9 @@ HTMLSlotElement::ChildrenInFlatTreeIfAssignmentIsSupported() {
 
 void HTMLSlotElement::DetachLayoutTree(const AttachContext& context) {
   if (SupportsAssignment()) {
-    // With incremental shadow dom, we need to make sure that assignment is
-    // recalcualted here. Unless that, assigned_nodes_ can be updated while in
-    // iterating it because node->LazyReattachIfAttached() might update
-    // assigned_nodes_. See http://crbug.com/843069#c8
     const HeapVector<Member<Node>>& flat_tree_children =
-        RuntimeEnabledFeatures::SlotInFlatTreeEnabled()
-            ? (RuntimeEnabledFeatures::IncrementalShadowDOMEnabled()
-                   ? AssignedNodes()
-                   : assigned_nodes_)
-            : distributed_nodes_;
+        RuntimeEnabledFeatures::SlotInFlatTreeEnabled() ? assigned_nodes_
+                                                        : distributed_nodes_;
     for (auto& node : flat_tree_children)
       node->LazyReattachIfAttached();
   }