Mojo: Simplify typemap usage.

mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h

 // 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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_TRAITS_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_TRAITS_H_
 
 #include <stdint.h>
 
 #include <string>
 
 #include "base/strings/string_piece.h"
 #include "mojo/public/cpp/bindings/struct_traits.h"
 #include "mojo/public/cpp/bindings/tests/struct_with_traits_impl.h"
 
 namespace mojo {
+namespace test {
+class StructWithTraits;
+class StructWithTraits_Reader;
+}
 
 template <>
 struct StructTraits<test::StructWithTraits, test::StructWithTraitsImpl> {
   // Deserialization to test::StructTraitsImpl.
   static bool Read(test::StructWithTraits_Reader r,
                    test::StructWithTraitsImpl* out);
 
   // Fields in test::StructWithTraits.
   // See src/mojo/public/interfaces/bindings/tests/test_native_types.mojom.
   static bool f_bool(const test::StructWithTraitsImpl& value) {
     return value.get_bool();
   }
 
   static uint32_t f_uint32(const test::StructWithTraitsImpl& value) {
     return value.get_uint32();
   }
 
   static uint64_t f_uint64(const test::StructWithTraitsImpl& value) {
     return value.get_uint64();
   }
 
   static base::StringPiece f_string(const test::StructWithTraitsImpl& value) {
     return value.get_string();
   }
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_TESTS_STRUCT_WITH_TRAITS_IMPL_TRAITS_H_