Add file header to heartbeat.js, props DrewAPicture, fixes #24723

git-svn-id: http://core.svn.wordpress.org/trunk@24654 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Logged out warnings:
- Close the iframe immediately on successful login.
- Catch iframe origin exceptions in WebKit when there is a server error or another page is loaded in the iframe.
See #23295.

git-svn-id: http://core.svn.wordpress.org/trunk@24655 1a063a9b-81f0-0310-95a4-ce76da25c4cd