Add slots for navigator.serviceWorker.installing and active.

These are not set by the embedder yet, so the value is always null.

BUG=379012

Review URL: https://codereview.chromium.org/335293003

git-svn-id: svn://svn.chromium.org/blink/trunk@176377 bbb929c8-8fbe-4397-9dbb-9b2b20218538