[browser] Guard capture grab ready for active web page. Contributes to JB#44721
[browser] Guard capture grab ready for active web page. Contributes to JB#44721

Also triggering capture is guarded only for active web pages.

sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser