Real-time collaboration: fix race condition in default polling provider.

See also: https://github.com/WordPress/wordpress-develop/pull/11067.
Developed in: https://github.com/WordPress/wordpress-develop/pull/11292.

Fixes #64887.
Props czarate, westonruter, mindctrl, peterwilsoncc, joefusco.

git-svn-id: https://develop.svn.wordpress.org/trunk@62064 602fd350-edb4-49c9-b593-d223f7449a82