Implement embed-size-denied

[camelburrito]

signal back ads/iframes when we couldn't expand on their embed-resize request. Send a postMessage with embed-size-denied (when the runtime can't resize)so that they can provide their own overflow element within and repeat request on tap. In that case AMP will definitely allow size change due to user action.

This would be useful for both: iframes and ads

CC @dvoytenko, @cramforce

[dvoytenko]

We may also want to have the opposite message: embed-size-changed or such. This is because we automatically resize when an element gets focused.

[camelburrito]

how about we give them a callback in this function
https://github.com/ampproject/amphtml/blob/master/3p/integration.js#L144

function triggerResizeRequest(width, height, callback) {
  nonSensitiveDataPostMessage('embed-size', {
    width: width,
    height: height,
  });
  // observe for embed-size-denied and embed-size-changed and 
  callback(status);
}

[dvoytenko]

Here, it'd probably have to have two callbacks: one for success and one for denied. But I'm not sure it'd be actually convenient here. These kind of callbacks might be just easier to subscribe to once and keep the listener.