[sailfish-browser] Add missing property for WebUtils mock
[sailfish-browser] Add missing property for WebUtils mock

A warning was printed when running unit tests.

rojkov/sailfish-browser,sailfishos/sailfish-browser,alinelena/sailfish-browser,alinelena/sailfish-browser,sailfishos/sailfish-browser,sailfishos/sailfish-browser,rojkov/sailfish-browser,sailfishos/sailfish-browser,rojkov/sailfish-browser,alinelena/sailfish-browser,sailfishos/sailfish-browser,alinelena/sailfish-browser