It's a poor design where a site is allowed to throw up a dialog like that (also goes for the "this site wants to know your location! yes/no").

It should work like the popup blocker, just show a red cross somewhere, that some action was blocked, and if the user wants to, he can go and unblock it. The onus should be on the site owner to tell his user "if you want this additional functionality, please go and enable <this or that>".