[heap] Verify client heaps during shared GC

With --verify-heap verify all client heaps before and after a shared
GC. This ensures that the OLD_TO_SHARED remembered set is properly
filled for each client isolate.

Bug: v8:11708
Change-Id: I1506a419c7a91c5baa87ce251da9861d8ad9e066
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3857559Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82829}

src/heap/heap.cc

@@ -2434,12 +2434,30 @@ void Heap::PerformSharedGarbageCollection(Isolate* initiator,
     if (FLAG_concurrent_marking) {
       client->heap()->concurrent_marking()->Pause();
     }
+
+#ifdef VERIFY_HEAP
+    if (FLAG_verify_heap) {
+      // We don't really perform a GC here but need this scope for the nested
+      // SafepointScope inside Verify().
+      AllowGarbageCollection allow_gc;
+      HeapVerifier::VerifyHeap(client->heap());
+    }
+#endif  // VERIFY_HEAP
   });
 
   const GarbageCollector collector = GarbageCollector::MARK_COMPACTOR;
   PerformGarbageCollection(collector, gc_reason, nullptr);
 
   isolate()->global_safepoint()->IterateClientIsolates([](Isolate* client) {
+#ifdef VERIFY_HEAP
+    if (FLAG_verify_heap) {
+      // We don't really perform a GC here but need this scope for the nested
+      // SafepointScope inside Verify().
+      AllowGarbageCollection allow_gc;
+      HeapVerifier::VerifyHeap(client->heap());
+    }
+#endif  // VERIFY_HEAP
+
     if (FLAG_concurrent_marking &&
         client->heap()->incremental_marking()->IsMarking()) {
       client->heap()->concurrent_marking()->RescheduleJobIfNeeded();