Expose the {title, lang, body, icon} properties on Notification instances.

This moves us closer to supporting the latest draft of the Web Notification
API. This patch does not change existing behavior, it only extends.

BUG=318746

Review URL: https://codereview.chromium.org/66133003

git-svn-id: svn://svn.chromium.org/blink/trunk@162017 bbb929c8-8fbe-4397-9dbb-9b2b20218538