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

mojo/services/files/public/c/tests/mojio_impl_test_base.cc

Index: mojo/services/files/public/c/tests/mojio_impl_test_base.cc
diff --git a/mojo/services/files/public/c/tests/mojio_impl_test_base.cc b/mojo/services/files/public/c/tests/mojio_impl_test_base.cc
index 6b606a0105e3a5f8660ca553126cd15483c45549..e3aeba33f2b302347255c9d3c0f137973b10a56a 100644
--- a/mojo/services/files/public/c/tests/mojio_impl_test_base.cc
+++ b/mojo/services/files/public/c/tests/mojio_impl_test_base.cc
@@ -25,10 +25,10 @@ void MojioImplTestBase::SetUp() {
   mojo::files::FilesPtr files;
   application_impl()->ConnectToService("mojo:files", &files);
 
-  mojo::files::Error error = mojo::files::ERROR_INTERNAL;
+  mojo::files::Error error = mojo::files::Error::INTERNAL;
   files->OpenFileSystem(nullptr, mojo::GetProxy(&directory_), Capture(&error));
   MOJO_CHECK(files.WaitForIncomingResponse());
-  MOJO_CHECK(error == mojo::files::ERROR_OK);
+  MOJO_CHECK(error == mojo::files::Error::OK);
 }
 
 }  // namespace test