Remember and reuse derived map for external arrays

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 4d2c6b4ceaa8bcddcf796e033e98e507319da528..aff24ba90ddafa1589fc26c155fa00fcb60b6a17 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -324,8 +324,10 @@ Handle<Map> Factory::GetSlowElementsMap(Handle<Map> src) {
 }
 
 
-Handle<Map> Factory::NewExternalArrayElementsMap(Handle<Map> src) {
-  CALL_HEAP_FUNCTION(src->NewExternalArrayElementsMap(), Map);
+Handle<Map> Factory::GetExternalArrayElementsMap(
+    Handle<Map> src,
+    ExternalArrayType array_type) {
+  CALL_HEAP_FUNCTION(src->GetExternalArrayElementsMap(array_type), Map);
 }