Add ongamepadconnected and ongamepaddisconnected event handlers to the WindowEventHandlers interface mixin. This would enable support for window.ongamepad[dis]connected or document.body.ongamepad[dis]connected event handler attributes.
The gamepad[dis]connected events can be subscribed to using window.addEventListener() currently. This adds the ability to use window.ongamepad[dis]connected event handler attributes as well.