Move reconfiguration into the elements accessor

src/elements.h

Index: src/elements.h
diff --git a/src/elements.h b/src/elements.h
index bab02b86dec5eb730aae67fac3f52f9f702041c8..0051c2e9f41b94686849ec64d78d667b01c3e580 100644
--- a/src/elements.h
+++ b/src/elements.h
@@ -140,6 +140,10 @@ class ElementsAccessor {
 
   virtual void Set(FixedArrayBase* backing_store, uint32_t key,
                    Object* value) = 0;
+  virtual void Reconfigure(Handle<JSObject> object,
+                           Handle<FixedArrayBase> backing_store, uint32_t index,
+                           Handle<Object> value,
+                           PropertyAttributes attributes) = 0;
 
  protected:
   friend class SloppyArgumentsElementsAccessor;