Unity: add InputTracking to my script

I'm new to unity and 3D world developement so I'm following Unity's tutorial for beginners right here. And I'm in the section with title "Camera Nodes" where they gave me a c# script to attach it to the camera object.
I did attach it to the camera but the problem with the script when I debug it I get an error of undefined class:

The name 'InputTracking' does not exist in this context

And this is the script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class UpdateEyeAnchors : MonoBehaviour {

    GameObject[] eyes = new GameObject[2];
    string[] eyeAnchorNames = { "LeftEyeAnchor", "RightEyeAnchor" };  

    // Use this for initialization
    void Start () {

    }

    // Update is called once per frame
    void Update () {
        for (int i = 0; i < 2; ++i)
        {
            // If the eye anchor is no longer a child of us, don't use it
            if (eyes[i] != null && eyes[i].transform.parent != transform)
            {
                eyes[i] = null;
            }

            // If we don't have an eye anchor, try to find one or create one
            if (eyes[i] == null)
            {
                Transform t = transform.Find(eyeAnchorNames[i]);
                if (t)
                    eyes[i] = t.gameObject;

                if (eyes[i] == null)
                {
                    eyes[i] = new GameObject(eyeAnchorNames[i]);
                    eyes[i].transform.parent = gameObject.transform;
                }
            }

            // Update the eye transform
            eyes[i].transform.localPosition = InputTracking.GetLocalPosition((VRNode)i);//--the error is this line
            eyes[i].transform.localRotation = InputTracking.GetLocalRotation((VRNode)i);//--and this one
        }
    }
}

This is the link of the class InputTracking in Unity's API, but I don't know how to add it to my script :)
Thanks in advance.