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
Remove unnecessary 'const's

Noticed here https://codereview.chromium.org/538333002/ .
Something's wrong between gn/gyp because this doesn't happen
in gyp, but remove these consts anyway as they're unnecessary.

R=thakis@chromium.org
BUG=335824

Review URL: https://codereview.chromium.org/551863004

git-svn-id: svn://svn.chromium.org/blink/trunk@181584 bbb929c8-8fbe-4397-9dbb-9b2b20218538