Display drupal 7 site in iframe?
I put this towards the end of settings.php (Drupal 7). It worked for me:

$conf['x_frame_options'] = 'ALLOW-FROM .facebook.com';


(I also use SSL/https (Let's Encrypt) on this site.)

x_frame_options don't allow for multiple values, i think. So 'allow-from domain.com' might disallow SAMEORIGIN. From this post it looks like the module Security Kit allow for setting Allow-From dynamically based on the current page request. I haven't tested it myself.