Update definition of `WebAuthenticationBroker` class for Windows Phone 8.1
Update definition of `WebAuthenticationBroker` class for Windows Phone 8.1

See: https://msdn.microsoft.com/en-us/library/windows/apps/xaml/windows.security.authentication.web.webauthenticationbroker.aspx

nobuoka/DefinitelyTyped,nobuoka/DefinitelyTyped