Mojo: Support enums as map keys in WTF

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

 // Copyright 2016 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;
 
 struct TestWTFCodeGeneration {
   string str;
   string? nullable_str;
   array<string> strs;
   array<string?> nullable_strs;
   array<array<int32>> arrays;
   array<bool> bools;
   array<handle<message_pipe>> handles;
   map<string, string?> str_map;
   map<int32, array<int32>> array_map;
   map<int32, handle<message_pipe>> handle_map;
   array<map<string, string?>> str_maps;
 };
 
 union TestWTFCodeGeneration2 {
   string str;
   array<string> strs;
   map<string, string?> str_map;
 };
 
 struct TestWTFStruct {
+  enum NestedEnum {
+    E0,
+    E1,
+  };
   string str;
   int32 integer;
 };
 
 interface TestWTF {
+  enum NestedEnum {
+    E0,
+    E1,
+  };
   EchoString(string? str) => (string? str);
   EchoStringArray(array<string?>? arr) => (array<string?>? arr);
   EchoStringMap(map<string, string?>? str_map)
       => (map<string, string?>? str_map);
 };
+
+enum TopLevelEnum {
+  E0,
+  E1,
+};