Making MigrateToMap() a single bottleneck for all migrations except slow-to-fast case.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 23924ca3979919882200fd3632aa962920e58ce0..e1587af290adadea8ca1966b4c151a6f50f901c7 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2632,6 +2632,11 @@ 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);
+
   static void UpdateAllocationSite(Handle<JSObject> object,
                                    ElementsKind to_kind);