If Supplement has a WeakMember to the corresponding Supplementable object, we can remove a bunch of LifecycleObservers (because the Supplement object can get the Supplementable object without observing the Supplementable object). We should add the WeakMember.

template<typename T>
class Supplement : public GarbageCollectedMixin {
  WeakMember<T> m_host;  // We should add this.
};