Introduce FieldIndex to unify and abstract property/field offset

src/string-stream.cc

Index: src/string-stream.cc
diff --git a/src/string-stream.cc b/src/string-stream.cc
index 480b788adcb9cd65d5c5fec1b32a8cc8452ce9e9..fb73719492597880bf57091a04918156a6e350ea 100644
--- a/src/string-stream.cc
+++ b/src/string-stream.cc
@@ -348,7 +348,8 @@ void StringStream::PrintUsingMap(JSObject* js_object) {
           key->ShortPrint();
         }
         Add(": ");
-        Object* value = js_object->RawFastPropertyAt(descs->GetFieldIndex(i));
+        FieldIndex index = FieldIndex::ForDescriptor(map, i);
+        Object* value = js_object->RawFastPropertyAt(index);
         Add("%o\n", value);
       }
     }