cppgc: Fix missing object start bitmap entry for filler

https://crrev.com/c/3218150 introduced a bug where we would create a
filler entry without updating the object start bitmap.

Bug: v8:12295
Change-Id: Ic39cea54d2e0e8297fe58eb1e5b22d787874c565
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218066
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77370}

src/heap/cppgc/object-allocator.h

@@ -166,7 +166,10 @@ void* ObjectAllocator::AllocateObjectOnSpace(NormalPageSpace& space,
   if (!lab_allocation_will_succeed &&
       (current_lab_size >= (size + kPaddingSize))) {
     void* filler_memory = current_lab.Allocate(kPaddingSize);
-    Filler::CreateAt(filler_memory, kPaddingSize);
+    auto& filler = Filler::CreateAt(filler_memory, kPaddingSize);
+    NormalPage::From(BasePage::FromPayload(&filler))
+        ->object_start_bitmap()
+        .SetBit<AccessMode::kAtomic>(reinterpret_cast<ConstAddress>(&filler));
     lab_allocation_will_succeed = true;
   }
   if (lab_allocation_will_succeed) {