[resize-observer] isActive missing from ResizeObservation interface

https://drafts.csswg.org/resize-observer-1/#dom-resizeobservation-isactive descrives a isActive() method for ResizeObservation, but it's not in the interface definition:

interface ResizeObservation {
    constructor(Element target, ResizeObserverBoxOptions observedBox);
    readonly attribute Element target;
    readonly attribute ResizeObserverBoxOptions observedBox;
    readonly attribute FrozenArray<ResizeObserverSize> lastReportedSizes;
};

isActive is used in https://drafts.csswg.org/resize-observer-1/#gather-active-observations-at-depth:

If observation.isActive() is true

where observation is a ResizeObservation.

Start with the ResizeObservation interface definition shown in the issue and compare it with the linked Resize Observer specification references. Update the interface so it reflects the documented isActive() method, then verify that the definition matches the specification's use of observation.isActive().