A class conforming to the WKNavigationDelegate protocol can provide
methods for tracking progress for main frame navigations and for deciding
policy for main frame and subframe navigations.
A class conforming to the WKScriptMessageHandlerWithReply protocol provides a
method for receiving messages from JavaScript running in a webpage and replying to them asynchronously.
A class conforming to the WKURLSchemeHandler protocol provides methods for
loading resources with URL schemes that WebKit doesn't know how to handle itself.