[heap] Reset pending object in final atomic pause

Not needed for correctness but this avoids adding the pending object
to the on_hold worklist.

Bug: v8:10315
Change-Id: Ide910cee37a4069c71c4046c32fa9f663265775e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2424137Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70085}

src/heap/mark-compact.cc

@@ -899,6 +899,11 @@ void MarkCompactCollector::Prepare() {
     heap_->safepoint()->IterateLocalHeaps(
         [](LocalHeap* local_heap) { local_heap->FreeLinearAllocationArea(); });
   }
+
+  // All objects are guaranteed to be initialized in atomic pause
+  heap()->new_lo_space()->ResetPendingObject();
+  DCHECK_EQ(heap()->new_space()->top(),
+            heap()->new_space()->original_top_acquire());
 }
 
 void MarkCompactCollector::FinishConcurrentMarking(