Mojo C++ bindings: support enum validation.

mojo/public/interfaces/bindings/tests/test_unions.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module mojo.test;
 
 enum AnEnum {
   FIRST, SECOND
 };
 
+[Extensible=True]
+enum AnExtensibleEnum {
+  FIRST, SECOND, THIRD
+};
+
 union PodUnion {
   int8   f_int8;
   int8   f_int8_other;
   uint8  f_uint8;
   int16  f_int16;
   uint16 f_uint16;
   int32  f_int32;
   uint32 f_uint32;
   int64  f_int64;
   uint64 f_uint64;
   float  f_float;
   double f_double;
   bool   f_bool;
   AnEnum f_enum;
+  AnExtensibleEnum f_extensible_enum;
 };
 
 union ObjectUnion {
   int8   f_int8;
   string f_string;
   DummyStruct f_dummy;
   DummyStruct? f_nullable;
   array<int8> f_array_int8;
   map<string, int8> f_map_int8;
   PodUnion f_pod_union;
@@ skipping 52 matching lines @@
 };
 
 union OldUnion {
   int8 f_int8;
 };
 
 union NewUnion {
   int8 f_int8;
   int16 f_int16;
 };