Add missing checks for failure after AddElementsTransition.

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 10740)
+++ src/objects.h	(working copy)
@@ -4746,8 +4746,8 @@
 
   // Adds an entry to this map's descriptor array for a transition to
   // |transitioned_map| when its elements_kind is changed to |elements_kind|.
-  MaybeObject* AddElementsTransition(ElementsKind elements_kind,
-                                     Map* transitioned_map);
+  MUST_USE_RESULT MaybeObject* AddElementsTransition(
+      ElementsKind elements_kind, Map* transitioned_map);
 
   // Returns the transitioned map for this map with the most generic
   // elements_kind that's found in |candidates|, or null handle if no match is