WebServiceWorkerProxy instances can be unwrapped to ServiceWorkers.

This does not change behavior because Chrome's service worker
dispatcher always mints new WebServiceWorker instances, and
WebServiceWorker::proxy returns null.

BUG=361907

Review URL: https://codereview.chromium.org/306133002

git-svn-id: svn://svn.chromium.org/blink/trunk@175219 bbb929c8-8fbe-4397-9dbb-9b2b20218538