[DevTools] Recursively compute the bounding rect of the roots (#34629)

It's possible for the children to overflow the bounding rect of the root
in general when they overflow in the DOM. However even when it doesn't
overflow in the DOM, the bounding rect of the root can shrink while the
content is suspended. In fact, it's very likely.

Originally I thought we didn't need to consider this recursively because
document scrolling takes absolute positioned content into account but
because we're using nested overflow scrolling, we have to manually
compute this.