Decouple root visitors from object visitors.

src/snapshot/serializer.h

Index: src/snapshot/serializer.h
diff --git a/src/snapshot/serializer.h b/src/snapshot/serializer.h
index 1af259db5730588cf61f4264bdf643ab36156489..69f92a7ef1c13897c2229fa2368432e850a11edc 100644
--- a/src/snapshot/serializer.h
+++ b/src/snapshot/serializer.h
@@ -156,7 +156,7 @@ class Serializer : public SerializerDeserializer {
   virtual void SerializeObject(HeapObject* o, HowToCode how_to_code,
                                WhereToPoint where_to_point, int skip) = 0;
 
-  void VisitPointers(Object** start, Object** end) override;
+  void VisitRootPointers(Root root, Object** start, Object** end) override;
 
   void PutRoot(int index, HeapObject* object, HowToCode how, WhereToPoint where,
                int skip);