Refactor embedded pointer visitors for the serializer

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index b423cd427994ec919f328da4b379fdf7b3a7c052..e71df5ad92af08c6a03f3e6c1161d1e460e7c761 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7890,6 +7890,8 @@ class ObjectVisitor BASE_EMBEDDED {
   // may be modified on return.
   virtual void VisitExternalReferences(Address* start, Address* end) {}
 
+  virtual void VisitExternalReference(RelocInfo* rinfo);
+
   inline void VisitExternalReference(Address* p) {
     VisitExternalReferences(p, p + 1);
   }