Further clean-ups in the Blink Web Notification implementation.

This patch switches the WebNotification API interface to use WebPrivatePtr,
renames various cancel() methods to close(), fixes IWYU in various places
and does more general clean-ups now that the legacy API is gone.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/194813005

git-svn-id: svn://svn.chromium.org/blink/trunk@169045 bbb929c8-8fbe-4397-9dbb-9b2b20218538