[snapshot] fix serialization description

R=jgruber@chromium.org

Change-Id: I1b3792dd678757eed8739de1f1c37c77838c52d9
Reviewed-on: https://chromium-review.googlesource.com/1125850Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54209}

src/snapshot/serializer.cc

@@ -293,11 +293,11 @@ void Serializer<AllocatorT>::PutBackReference(HeapObject* object,
   DCHECK(allocator()->BackReferenceIsAlreadyAllocated(reference));
   switch (reference.space()) {
     case MAP_SPACE:
-      sink_.PutInt(reference.map_index(), "BackRefChunkOffset");
+      sink_.PutInt(reference.map_index(), "BackRefMapIndex");
       break;
 
     case LO_SPACE:
-      sink_.PutInt(reference.large_object_index(), "BackRefChunkOffset");
+      sink_.PutInt(reference.large_object_index(), "BackRefLargeObjectIndex");
       break;
 
     default: