We should not GC allocate in the constructors of GC-allocated objects
because a new allocation might trigger garbage collection, which in
turn might access not fully initialized objects.
In FetchAlgorithms, it is common for callbacks to capture realms. This
can indirectly keep objects alive that hold FetchController with these
callbacks. This creates a cyclic dependency. However, when
JS::HeapFunction is used, this is not a problem, as captured by
callbacks values do not create new roots.
