Clarify "dispatch action chain" behavior when executed within a frame

There is currently a difference between Firefox and Chrome / Safari when actions are performed within an iframe. While Firefox uses the iframe's visual viewport as reference to dispatch the actions, the top-level browsing context's visible viewport is actually used by Chrome and Safari.

As per definition of the visual viewport on MDN:

The VisualViewport interface of the Visual Viewport API represents the visual viewport for a given window. For a page containing iframes, each iframe, as well as the containing page, will have a unique window object. Each window on a page will have a unique VisualViewport representing the properties associated with that window.

IMO this means that we do it right in Firefox, but it's not clear given that the specification is quite unclear.

I noticed that issue when trying to fix some wpt tests around input events (see https://github.com/web-platform-tests/wpt/pull/48147).

I also landed a test which demonstrates the difference in behavior:
https://github.com/web-platform-tests/wpt/pull/48123

Start with the issue's comparison of Firefox, Chrome, and Safari behavior and the linked MDN VisualViewport definition. Review web-platform-tests PRs 48147 and 48123 to understand the demonstrated iframe behavior, then clarify the WebDriver specification so the expected dispatch action chain behavior is explicit.