For c++, Generate enum classes instead of enum from mojom.

mojo/public/cpp/bindings/tests/request_response_unittest.cc

 // 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.
 
 #include "mojo/public/cpp/bindings/binding.h"
 #include "mojo/public/cpp/environment/environment.h"
 #include "mojo/public/cpp/test_support/test_utils.h"
 #include "mojo/public/cpp/utility/run_loop.h"
 #include "mojo/public/interfaces/bindings/tests/sample_import.mojom.h"
 #include "mojo/public/interfaces/bindings/tests/sample_interfaces.mojom.h"
@@ skipping 122 matching lines @@
   ReadTextMessage(pipe2.handle0.get(), &value);
 
   EXPECT_EQ(std::string("hello"), value);
 }
 
 TEST_F(RequestResponseTest, EchoEnum) {
   sample::ProviderPtr provider;
   ProviderImpl provider_impl(GetProxy(&provider));
 
   sample::Enum value;
-  provider->EchoEnum(sample::ENUM_VALUE, EnumRecorder(&value));
+  provider->EchoEnum(sample::Enum::VALUE, EnumRecorder(&value));
 
   PumpMessages();
 
-  EXPECT_EQ(sample::ENUM_VALUE, value);
+  EXPECT_EQ(sample::Enum::VALUE, value);
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace mojo