Function IsValid

test-app/runtime/src/main/cpp/include/cppgc/persistent.h:229–234

  }

  bool IsValid() const {
    // Ideally, handling kSentinelPointer would be done by the embedder. On the
    // other hand, having Persistent aware of it is beneficial since no node
    // gets wasted.
    return GetValue() != nullptr && GetValue() != kSentinelPointer;
  }

  void Assign(T* ptr) {
    if (IsValid()) {