Refactor embedded pointer visitors for the serializer

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 2a07ca360b1f61d9f33f3b19c89691c711144208..238fe1e78e8b64245e97156582b5b7c8674bc65a 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7718,6 +7718,10 @@ void ObjectVisitor::VisitEmbeddedPointer(RelocInfo* rinfo) {
   VisitPointer(rinfo->target_object_address());
 }
 
+void ObjectVisitor::VisitExternalReference(RelocInfo* rinfo) {
+  Address* p = rinfo->target_reference_address();
+  VisitExternalReferences(p, p + 1);
+}
 
 void Code::InvalidateRelocation() {
   set_relocation_info(GetHeap()->empty_byte_array());