DevTools: Use jsdocs @template for custom Array methods and remove corresponding casts.

BUG=None
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/19598003

git-svn-id: svn://svn.chromium.org/blink/trunk@154475 bbb929c8-8fbe-4397-9dbb-9b2b20218538