Disallow recursive recalc slot assignment

This CL is separated out from the wip CL, https://crrev.com/c/1350441,
where recursive recalc slot assignment causes the tricky bug which is
hard to debug.

It would be better to detect recursive recalc slot assignment, which
shouldn't happen.

Change-Id: I07d2d22ea8e083ef1de5134d6879cb1a141ce183

BUG=906494

Change-Id: I07d2d22ea8e083ef1de5134d6879cb1a141ce183
Reviewed-on: https://chromium-review.googlesource.com/c/1354727Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612098}

third_party/blink/renderer/core/dom/slot_assignment.cc

@@ -9,6 +9,7 @@
 #include "third_party/blink/renderer/core/dom/node_traversal.h"
 #include "third_party/blink/renderer/core/dom/shadow_root.h"
 #include "third_party/blink/renderer/core/dom/slot_assignment_engine.h"
+#include "third_party/blink/renderer/core/dom/slot_assignment_recalc_forbidden_scope.h"
 #include "third_party/blink/renderer/core/html/forms/html_opt_group_element.h"
 #include "third_party/blink/renderer/core/html/forms/html_select_element.h"
 #include "third_party/blink/renderer/core/html/html_details_element.h"
@@ -226,6 +227,9 @@ void SlotAssignment::RecalcAssignment() {
 #if DCHECK_IS_ON()
   DCHECK(!owner_->GetDocument().IsSlotAssignmentRecalcForbidden());
 #endif
+  // To detect recursive RecalcAssignment, which shouldn't happen.
+  SlotAssignmentRecalcForbiddenScope forbid_slot_recalc(owner_->GetDocument());
+
   needs_assignment_recalc_ = false;
 
   for (Member<HTMLSlotElement> slot : Slots())