In `Custom_Image_Header`:

* In `->process_default_headers()`, remove check on non-existent `$headers` property. Introduced in [13403]. 
* Declare `$updated` as a property.

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30187 602fd350-edb4-49c9-b593-d223f7449a82