Convert GN visibility to be a list.

GN visibility currently allows either string or list types, but this is causing
some problems for some templates. I'm going to require it to be lists, so am
changing all callers before pushing the new binary.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/556583002

Patch from Brett Wilson <brettw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23789 ce2b1a6d-e550-0410-aec6-3dcde31c8c00