Theme Customizer: Load the preview iframe with the same scheme as the admin to...

Theme Customizer: Load the preview iframe with the same scheme as the admin to prevent security warnings. props ocean90. fixes #20137, see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20051 1a063a9b-81f0-0310-95a4-ce76da25c4cd