Intervention: Ignore clicks on iframes that have moved in the last 200 ms

[johannhof]

Original Interventions issue: WICG/interventions#19,
Chromestatus: https://chromestatus.com/feature/5406180975312896
Explainer: https://docs.google.com/document/d/1qzgHiruzTvt0_-rfe7suZRCe2KhSJxiBp28ZTLd9Xls/edit

Chrome (only) is shipping an intervention where clicks on iframes that are using IntersectionObserver V2 and have moved within the last 200ms are ignored. I'm filing this issue in HTML as a follow-up to the interventions issue to track this behavior and discuss what would be needed to have it fully supported in the spec. My guess is that we'd mostly need to ensure that the user agent may ignore events at their own volition.

cc @szager-chromium, @domenic