Add the navigator.serviceWorker.waiting property.

The value of this property will be null because it is not being set by
the embedder yet.

BUG=379012

Review URL: https://codereview.chromium.org/313553002

git-svn-id: svn://svn.chromium.org/blink/trunk@175520 bbb929c8-8fbe-4397-9dbb-9b2b20218538