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.

Merges [24908] to the 3.6 branch.

props azaozz.
fixes #24894.



git-svn-id: https://core.svn.wordpress.org/branches/3.6@24909 1a063a9b-81f0-0310-95a4-ce76da25c4cd