[heap] DCHECK that generational barrier is run on main thread

The generational barrier isn't supported on the background thread at the
moment. Make sure it isn't used on such threads by accident.

Bug: v8:13203
Change-Id: I5577f3802c1aba246955519c8c778fa741d56d96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3840300
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82635}

src/heap/heap.cc

@@ -7224,6 +7224,7 @@ void Heap::CombinedGenerationalAndSharedBarrierSlow(HeapObject object,
   MemoryChunk* value_chunk = MemoryChunk::FromHeapObject(value);
 
   if (value_chunk->InYoungGeneration()) {
+    DCHECK_NULL(LocalHeap::Current());
     Heap::GenerationalBarrierSlow(object, slot, value);
 
   } else {