Rename navigator.serviceWorker.current to controller.

Spec: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#navigator-service-worker-controller

BUG=363967

Review URL: https://codereview.chromium.org/301103003

git-svn-id: svn://svn.chromium.org/blink/trunk@175237 bbb929c8-8fbe-4397-9dbb-9b2b20218538