Theme Customizer: Bind the preview iframe load event when triggering refresh...

Theme Customizer: Bind the preview iframe load event when triggering refresh instead of when creating the iframe. see #19910.

The loaded event should only fire when we've actually triggered a refresh. If loaded is bound when the iframe is created, sometimes the blank iframe will fire a 'load' event before a refresh is triggered (this occurred with relative frequency in firefox). By binding the loaded event in the refresh function, we prevent this from occurring.

git-svn-id: https://core.svn.wordpress.org/trunk@20049 1a063a9b-81f0-0310-95a4-ce76da25c4cd