To check if the control has any potential tabs and headers, added:
* WP_Customize_Image_Control->prepare_control()
* WP_Customize_Header_Image_Control->prepare_control()
* WP_Customize_Header_Image_Control->default_headers
* WP_Customize_Header_Image_Control->uploaded_headers
git-svn-id: https://develop.svn.wordpress.org/trunk@21383 602fd350-edb4-49c9-b593-d223f7449a82