Note: This issue first regresses in 2017.1.0b7. In this editor, UnityEngine.XR does not exist yet - it is still named UnityEngine.VR. In this PR we use the acronym XR for consistency.

Expected Results:
Using either Oculus or OpenVR SDK, UnityEngine.XR.InputTracking events should fire correctly for TrackingReferences, Head, Left Eye, Right Eye, Center Eye, and peripherals. UnityEngine.XR.InputTracking.GetNodeStates() should return all nodes that are currently added to the system.

Observed Results:
- Using Oculus SDK: Right Eye, Center Eye, and Head nodes do not trigger UnityEngine.XR.InputTracking events or show up in UnityEngine.XR.InputTracking.GetNodeStates()
- Using OpenVR SDK:
--- Right Eye does not trigger UnityEngine.XR.InputTracking events or show up in UnityEngine.XR.InputTracking.GetNodeStates()
--- TrackingLost: Center Eye and TrackingAcquired: Head events fire every frame (Vive/Rift HW independent).

Steps to Reproduce:
- Open the attached project in 2017.1.0b7 or later
- In PlayerSettings, select either Oculus or OpenVR as your XR SDK.
- Open the scene asset "NewScene"
- Press Play
--- On the UI panel you can see UnityEngine.XR.InputTracking events as they occur. There is a bool checkbox that will stop events from reporting after the first 15 on the GameObject Canvas->Node Events->(Component) NodeEvents
--- The cubes that represent XRNodes are driven by UnityEngine.XR.InputTracking.GetNodeStates(). The script for this is located on the GameObject GetNodeStates.