Speculative fix for wxWebView unit tests on OSX.

Use stringByEvaluatingJavaScriptFromString as it is guaranteed to return an NSString.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73931 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775