Remember and reuse derived map for external arrays

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 26d2246aaea0c01ae2733b160961174834cb0058..d7f5246e9104423aaf27bb60f7bf4312a8a969dc 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -2775,8 +2775,8 @@ void PrepareExternalArrayElements(i::Handle<i::JSObject> object,
       elements->map() != i::Heap::MapForExternalArrayType(array_type);
   if (force_unique_map) {
     i::Handle<i::Map> external_array_map =
-        i::Factory::NewExternalArrayElementsMap(
-            i::Handle<i::Map>(object->map()));
+        i::Factory::GetExternalArrayElementsMap(
+            i::Handle<i::Map>(object->map()), array_type);
     object->set_map(*external_array_map);
   }
   object->set_elements(*array);