API REVIEW: RT-23888, Make PopupFeatures and PromptData final

[Peter Zhelezniakov]

> What is the gain of making them final and what is the loss of not
> making them final?

I was going to make them final mostly for consistency with the rest of
the platform. Also to prevent bugs e.g. when instances were used as hash
keys, although I don't expect them to be used that way.

Instances of those classes are used solely to carry data passed into
WebEngine callbacks. No API method accepts them as parameters. So having
them final seems a fine choice from API standpoint.

Thanks!
-- 
Peter