Mojo C++ bindings: custom type mapping of map

mojo/public/interfaces/bindings/tests/struct_with_traits.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 NestedStructWithTraits {
   int32 value;
 };
 
 struct StructWithTraits {
   bool f_bool;
   uint32 f_uint32;
   uint64 f_uint64;
   string f_string;
   string f_string2;
   array<string> f_string_array;
   NestedStructWithTraits f_struct;
   array<NestedStructWithTraits> f_struct_array;
+  map<string, NestedStructWithTraits> f_struct_map;
 };
 
 struct PassByValueStructWithTraits {
   handle f_handle;
 };
 
 interface TraitsTestService {
   EchoStructWithTraits(StructWithTraits s) => (StructWithTraits passed);
 
   EchoPassByValueStructWithTraits(PassByValueStructWithTraits s) =>
       (PassByValueStructWithTraits passed);
 };