Fix Object.freeze on dictionary-backed arrays to properly freeze elements

Follow-up to r14758: slightly rearranges JSObject::Freeze() to avoid duplicating
code while still retaining proper dictionary elements storage behavior.

Also fix a lint error.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/15737018

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14759 ce2b1a6d-e550-0410-aec6-3dcde31c8c00