Heartbeat: Reduce the heartbeat from 120 sec to 100 sec when the window doesn't have the focus, to be shorter than the post lock expiration window.

props azaozz.
fixes #24894.



git-svn-id: https://core.svn.wordpress.org/trunk@24908 1a063a9b-81f0-0310-95a4-ce76da25c4cd