Commit ed88cbe removed `new WebViewWebKit` on macOS because wx 3.3
dropped the no-arg wxWebViewWebKit constructor, falling through to
wxWebView::New(). That bypassed the WebViewWebKit destructor that calls
RemoveScriptMessageHandler("wx"), reintroducing the WebKit teardown bug
the subclass was added to fix.
Restore the macOS-specific subclass path by adding a constructor that
forwards to the wx 3.3 wxWebViewConfiguration-based ctor.
d62aa42e61