From https://bugs.webkit.org/show_bug.cgi?id=78085#c8 Document has resumeActiveDOMObjects() and suspendActiveDOMObjects(), which accesses Page's supplemental objects. These special case should be generalized away.
I wonder if we should have something more like a "PageObserver" than an ActiveDOMObject. We can add callbacks as needed by use cases.
This doesn't appear to be necessary.