Convert GN visibility to lists.

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.

Review URL: https://codereview.chromium.org/550333003

git-svn-id: svn://svn.chromium.org/blink/trunk@181583 bbb929c8-8fbe-4397-9dbb-9b2b20218538