Add MediaKeySession.closed attribute

This attribute returns a promise that is resolved with undefined when
the session is closed.

BUG=358271
TEST=new layout test passes

Review URL: https://codereview.chromium.org/398583005

git-svn-id: svn://svn.chromium.org/blink/trunk@179269 bbb929c8-8fbe-4397-9dbb-9b2b20218538