[resize-observer] isActive missing from ResizeObservation interface

[Loirooriol]

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.