JFR: Unify handling of all OopStorage instances in LeakProfiler root processing

We want it to be as convenient as possible for the runtime to add new OopStorage instances. Therefore, rather than picking specific OopStorages during LeakProfiler root processing, the OopStorageSet::strong_iterator() should be used instead. That way, once a new OopStorage is added to the OopStorageSet, it is automatically plugged in to the JFR LeakProfiler.