[Extensions] Don't allow built-in extensions code to be overridden
BUG=546677

Review URL: https://codereview.chromium.org/1417513003

Cr-Commit-Position: refs/heads/master@{#356654}
  explicit TestNatives(const base::Closure& quit_closure)
      : quit_closure_(quit_closure) {}

  explicit TestNatives(const base::Closure& quit_closure)
      : quit_closure_(quit_closure) {}
