Instantiate script provider when navigator.serviceWorker is accessed

Currently we do this lazily when register/unregister
method is called, but now that ServiceWorkerContainer is
a container class and also an event target, we should
set up script provider when ServiceWorkerContainer is instantiated.

BUG=349919

Review URL: https://codereview.chromium.org/190873002

git-svn-id: svn://svn.chromium.org/blink/trunk@168751 bbb929c8-8fbe-4397-9dbb-9b2b20218538