Remember and reuse derived map for external arrays

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index ca065902c3c84bcd7a666f68ca445646f5ac857d..f40b27697cf44603635589085854e7a6a565c87e 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1724,7 +1724,8 @@ bool DescriptorArray::IsProperty(int descriptor_number) {
 
 bool DescriptorArray::IsTransition(int descriptor_number) {
   PropertyType t = GetType(descriptor_number);
-  return t == MAP_TRANSITION || t == CONSTANT_TRANSITION;
+  return t == MAP_TRANSITION || t == CONSTANT_TRANSITION ||
+      t == EXTERNAL_ARRAY_TRANSITION;
 }
 
 
@@ -2868,21 +2869,6 @@ MaybeObject* Map::GetSlowElementsMap() {
 }
 
 
-MaybeObject* Map::NewExternalArrayElementsMap() {
-  // TODO(danno): Special case empty object map (or most common case)
-  // to return a pre-canned pixel array map.
-  Object* obj;
-  { MaybeObject* maybe_obj = CopyDropTransitions();
-    if (!maybe_obj->ToObject(&obj)) return maybe_obj;
-  }
-  Map* new_map = Map::cast(obj);
-  new_map->set_has_fast_elements(false);
-  new_map->set_has_external_array_elements(true);
-  COUNTERS->map_to_external_array_elements()->Increment();
-  return new_map;
-}
-
-
 ACCESSORS(Map, instance_descriptors, DescriptorArray,
           kInstanceDescriptorsOffset)
 ACCESSORS(Map, code_cache, Object, kCodeCacheOffset)