The "push" event should inherit from ExtendableEvent.

This makes it possible for developers to call waitUntil() on the
event, in case its lifetime should be extended.

Specification link (Push API):
  https://w3c.github.io/push-api/#the-push-event


BUG=430888

Review URL: https://codereview.chromium.org/716203003

git-svn-id: svn://svn.chromium.org/blink/trunk@185374 bbb929c8-8fbe-4397-9dbb-9b2b20218538