Theme Customizer: Make dropdown-pages a native customize control. see #19910.

While the customize_render_control- action has been removed, we could still accomplish this with the customize_render_setting action. That said, in this case, avoiding native integration was a matter of minor semantics that ended in the same result.

git-svn-id: https://core.svn.wordpress.org/trunk@20254 1a063a9b-81f0-0310-95a4-ce76da25c4cd