Decouple root visitors from object visitors.

src/snapshot/startup-serializer.h

Index: src/snapshot/startup-serializer.h
diff --git a/src/snapshot/startup-serializer.h b/src/snapshot/startup-serializer.h
index 4a597e6a329a023108e8677507a3ddce5c81e748..223e1c7bff3b34c67d0f05ea281ce2ad8c077430 100644
--- a/src/snapshot/startup-serializer.h
+++ b/src/snapshot/startup-serializer.h
@@ -57,7 +57,7 @@ class StartupSerializer : public Serializer {
 
   // The StartupSerializer has to serialize the root array, which is slightly
   // different.
-  void VisitPointers(Object** start, Object** end) override;
+  void VisitRootPointers(Root root, Object** start, Object** end) override;
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
   void Synchronize(VisitorSynchronization::SyncTag tag) override;