Provide utilities to help components to resolve risk conditions between layout and viewport events

[dvoytenko]

Currently it's left up to the component to resolve states between "layout" and "viewport" events and it's too easy to introduce a risk condition. In an example, amp-anim autoplays GIF when it enters viewport, but it needs to make sure that the image is ready. It's not too hard to implement it fully, but perhaps this can be made simpler.