Dispatch a 'close' event on a <dialog> when it closes.

As per the spec, the final step of closing a dialog is:
"Queue a task to fire a simple event named close at subject."
http://www.whatwg.org/specs/web-apps/current-work/multipage/commands.html#dom-dialog-close

BUG=276785

Review URL: https://chromiumcodereview.appspot.com/22836009

git-svn-id: svn://svn.chromium.org/blink/trunk@156765 bbb929c8-8fbe-4397-9dbb-9b2b20218538