use c++03 style enum scope

Followup to https://codereview.chromium.org/411973004

We haven't officially switched over to using c++11 everywhere
yet, so this patch replaces c++11 style enum scope with c++03 style scope.

BUG=392621

Review URL: https://codereview.chromium.org/526193002

Cr-Commit-Position: refs/heads/master@{#292902}
1 file changed