Remember and reuse derived map for external arrays

src/property.h

Index: src/property.h
diff --git a/src/property.h b/src/property.h
index c39fe41e75ddd99bc6b9a1cf52aebc34599f2b9f..7067535991d4b0bfc87282bf85d1b286a91c64ee 100644
--- a/src/property.h
+++ b/src/property.h
@@ -110,6 +110,16 @@ class MapTransitionDescriptor: public Descriptor {
       : Descriptor(key, map, attributes, MAP_TRANSITION) { }
 };
 
+class ExternalArrayTransitionDescriptor: public Descriptor {
+ public:
+  ExternalArrayTransitionDescriptor(String* key,
+                                    Map* map,
+                                    ExternalArrayType array_type)
+      : Descriptor(key, map, PropertyDetails(NONE,
+                                             EXTERNAL_ARRAY_TRANSITION,
+                                             array_type)) { }
+};
+
 // Marks a field name in a map so that adding the field is guaranteed
 // to create a FIELD descriptor in the new map.  Used after adding
 // a constant function the first time, creating a CONSTANT_FUNCTION