[runtime] Turn MigrateFastTo* into static helpers

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 08e42b45e47331a1f065bb64006397c165432c22..2c6e91b11057e1d06a29f7a1b2e871395b9750e9 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2498,11 +2498,6 @@ class JSObject: public JSReceiver {
   friend class JSReceiver;
   friend class Object;
 
-  static void MigrateFastToFast(Handle<JSObject> object, Handle<Map> new_map);
-  static void MigrateFastToSlow(Handle<JSObject> object,
-                                Handle<Map> new_map,
-                                int expected_additional_properties);
-
   // Used from Object::GetProperty().
   MUST_USE_RESULT static MaybeHandle<Object> GetPropertyWithFailedAccessCheck(
       LookupIterator* it);