Make NotificationEvent constructable.

This is possible now that Service Worker's ExtendableEvent
has been made constructable.

Specification:
  https://notifications.spec.whatwg.org/#service-worker-api


BUG=432527

Review URL: https://codereview.chromium.org/716143003

git-svn-id: svn://svn.chromium.org/blink/trunk@185371 bbb929c8-8fbe-4397-9dbb-9b2b20218538