Empty Array prototype elements protection needs to alert on length change.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index fb4e069ad4f029a04378283f2029befa469e50be..d10e64b6d129382ccdaf775f5c69b75387bab823 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1021,6 +1021,9 @@ class Isolate {
   // object prototype. Also ensure that changes to prototype chain between
   // Array and Object fire notifications.
   void UpdateArrayProtectorOnSetElement(Handle<JSObject> object);
+  void UpdateArrayProtectorOnSetLength(Handle<JSObject> object) {
+    UpdateArrayProtectorOnSetElement(object);
+  }
   void UpdateArrayProtectorOnSetPrototype(Handle<JSObject> object) {
     UpdateArrayProtectorOnSetElement(object);
   }