Add an example computing CLS including iframe

[npm1]

The CLS is defined in the non-normative section and it's not super clear to me how you compute it. In particular, the subframe weighting factor could change over time? (i.e. should it be computed on every frame, or how does one account for an iframe that shifts or moves after user interaction and hence becomes more/less visible within the viewport).

[skobes-chromium]

Chrome recomputes weighting factor on every frame. Agree an example would help here.