Implement the ServiceWorkerRegistration.showNotification() method.

This patch implements the showNotification() method by extending the
ServiceWorkerRegistration object, and calling through to the Platform
for requesting display of a persistent notification.

BUG=432527

Review URL: https://codereview.chromium.org/717273002

git-svn-id: svn://svn.chromium.org/blink/trunk@185326 bbb929c8-8fbe-4397-9dbb-9b2b20218538