LibWeb: Visit IntersectionObserverRegistration instead of using Handle

This fixes GC-leak caused by JS::Handle<IntersectionObserverver>
preventing an element that owns the handle from being deallocated.

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -128,6 +128,8 @@ void Element::visit_edges(Cell::Visitor& visitor)
  visitor.visit(m_custom_element_definition.ptr());
  for (auto& pseudo_element_layout_node : m_pseudo_element_nodes)
  visitor.visit(pseudo_element_layout_node);
+ for (auto& registered_intersection_observers : m_registered_intersection_observers)
+ visitor.visit(registered_intersection_observers.observer);
 }
 
 // https://dom.spec.whatwg.org/#dom-element-getattribute

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp

@@ -92,7 +92,7 @@ void IntersectionObserver::observe(DOM::Element& target)
  // property set to observer, a previousThresholdIndex property set to -1, and a previousIsIntersecting
  // property set to false.
  auto intersection_observer_registration = IntersectionObserverRegistration {
- .observer = JS::make_handle(*this),
+ .observer = *this,
  .previous_threshold_index = OptionalNone {},
  .previous_is_intersecting = false,
  };

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.h

@@ -23,7 +23,7 @@ struct IntersectionObserverInit {
 struct IntersectionObserverRegistration {
  // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserverregistration-observer
  // [A]n observer property holding an IntersectionObserver.
- JS::Handle<IntersectionObserver> observer;
+ JS::NonnullGCPtr<IntersectionObserver> observer;
 
  // https://www.w3.org/TR/intersection-observer/#dom-intersectionobserverregistration-observer
  // NOTE: Optional is used in place of the spec using -1 to indicate no previous index.