Theme Customizer: First pass at using postMessage for background color. Fix...

Theme Customizer: First pass at using postMessage for background color. Fix instance where preview.targetWindow would become inaccurate. Initialize setting values in customize-preview.js. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20123 1a063a9b-81f0-0310-95a4-ce76da25c4cd