heap: Add check for always_promote_young_mc

Add CHECK ensuring that the young generation is indeed empty for
pointers updating. This is necessary as otherwise iterating an
object may race with updating a slot in a Map for WasmStruct.

Bug: v8:12185
Change-Id: Id590cf267fedf95d97df2464a638352696ad53db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3260514Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77709}

src/heap/mark-compact.cc

@@ -4260,6 +4260,11 @@ void MarkCompactCollector::UpdatePointersAfterEvacuation() {
     CollectRememberedSetUpdatingItems(&updating_items, heap()->map_space(),
                                       RememberedSetUpdatingMode::ALL);
 
+    // In order to update pointers in map space at the same time as other spaces
+    // we need to ensure that young generation is empty. Otherwise, iterating
+    // to space may require a valid body descriptor for e.g. WasmStruct which
+    // races with updating a slot in Map.
+    CHECK(FLAG_always_promote_young_mc);
     CollectToSpaceUpdatingItems(&updating_items);
     updating_items.push_back(
         std::make_unique<EphemeronTableUpdatingItem>(heap()));