Notify VRDeviceProviders when there are pages listening for vrdisplayactivate

device/vr/vr_device_provider.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DEVICE_VR_VR_DEVICE_PROVIDER_H
 #define DEVICE_VR_VR_DEVICE_PROVIDER_H
 
 #include <vector>
 
 namespace device {
 
 class VRDevice;
 class VRServiceImpl;
 
 class VRDeviceProvider {
  public:
   VRDeviceProvider() {}
   virtual ~VRDeviceProvider() {}
 
   virtual void GetDevices(std::vector<VRDevice*>* devices) = 0;
 
   // If the VR API requires initialization that should happen here.
   virtual void Initialize() = 0;
 
   virtual void PollEvents() {}
+
+  virtual void SetListeningForActivate(bool listening) {}
 };
 
 }  // namespace device
 
 #endif  // DEVICE_VR_VR_DEVICE_PROVIDER_H