use c++03 style enum scope

Followup to https://codereview.chromium.org/473833002

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=402749

Review URL: https://codereview.chromium.org/528843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181202 bbb929c8-8fbe-4397-9dbb-9b2b20218538