Sync the alert() IDL with the spec

http://html5.org/r/8638
https://www.w3.org/Bugs/Public/show_bug.cgi?id=25686

This does not change the behavior of alert() or alert(undefined), it is
a pre-emptive change so that treating undefined as a missing optional
argument (per WebIDL) does not change the behavior.

BUG=335871

Review URL: https://codereview.chromium.org/296853010

git-svn-id: svn://svn.chromium.org/blink/trunk@174845 bbb929c8-8fbe-4397-9dbb-9b2b20218538