Theme Customizer: Do not attempt to grab the contents of the preview iframe to...

Theme Customizer: Do not attempt to grab the contents of the preview iframe to populate the window title. Reverts plugins repo revision 508947. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20250 1a063a9b-81f0-0310-95a4-ce76da25c4cd