[serializer] clean up root list items.

src/snapshot/serializer.cc

Index: src/snapshot/serializer.cc
diff --git a/src/snapshot/serializer.cc b/src/snapshot/serializer.cc
index f622a5b8d24a795d8411f1899085915f03ccb898..24041e3d93455136caabb91d1cf0258814c6d6c3 100644
--- a/src/snapshot/serializer.cc
+++ b/src/snapshot/serializer.cc
@@ -212,6 +212,11 @@ void Serializer::PutRoot(int root_index, HeapObject* object,
     PrintF("\n");
   }
 
+  // Assert that the first 32 root array items are a conscious choice. They are
+  // chosen so that the most common ones can be encoded more efficiently.
+  STATIC_ASSERT(Heap::kEmptyDescriptorArrayRootIndex ==
+                kNumberOfRootArrayConstants - 1);
+
   if (how_to_code == kPlain && where_to_point == kStartOfObject &&
       root_index < kNumberOfRootArrayConstants &&
       !isolate()->heap()->InNewSpace(object)) {