[ServiceWorker] Add missing method to Cache.idl.

The string version of Add() was unfortunately omitted in an earlier CL.

R=falken@chromium.org,jkarlin@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/458853002

git-svn-id: svn://svn.chromium.org/blink/trunk@179946 bbb929c8-8fbe-4397-9dbb-9b2b20218538