To implement https://bugs.openjdk.org/browse/JDK-8310160 we need to add an API that returns the reserved heap memory range. All collectors except ZGC set CollectedHeap::_reserved during init but only some of them expose it as an API - ParallelScavengeHeap::reserved_region(), GenCollectedHeap::reserved_region() and ShenandoahHeap:::reserved_region().
Instead of having each collector exposing an API we can instead have a common API CollectedHeap::reserved_region().