This method has no parameters.
A object that has these properties:
animationPromise: A Promise for the animation triggered by the layout pass, or Promise.wrap if there are no animations.
newEndIndex: If this value is null, there's no more work to do. Any other value means a new layout pass needs to start.
Your implementation can use this method to start animations and to check whether there needs to be another layout pass.
Minimum WinJS version