Isolate a presenting VR device from pages other than the one presenting.

device/vr/vr_device_manager.h

Index: device/vr/vr_device_manager.h
diff --git a/device/vr/vr_device_manager.h b/device/vr/vr_device_manager.h
index 15e6bc4ae25e6b1fc14fadc94607fe0e35b6735c..8c8e14c4fe49b161a4e83eac88e8e53210f7291a 100644
--- a/device/vr/vr_device_manager.h
+++ b/device/vr/vr_device_manager.h
@@ -32,14 +32,22 @@ class VRDeviceManager : public VRClientDispatcher {
   // Returns the VRDeviceManager singleton.
   static VRDeviceManager* GetInstance();
 
+  // Gets a VRDevice instance if the specified service is allowed to access it.
+  static VRDevice* GetAllowedDevice(VRServiceImpl* service, unsigned int index);
+
   // Adds a listener for device manager events. VRDeviceManager does not own
   // this object.
   void AddService(VRServiceImpl* service);
   void RemoveService(VRServiceImpl* service);
 
   DEVICE_VR_EXPORT mojo::Array<VRDisplayPtr> GetVRDevices();
-  DEVICE_VR_EXPORT VRDevice* GetDevice(unsigned int index);
 
+  // Manage presentation to only allow a single service and device at a time.
+  bool RequestPresent(VRServiceImpl* service, unsigned int index);
+  void ExitPresent(VRServiceImpl* service, unsigned int index);
+  void SubmitFrame(VRServiceImpl* service, unsigned int index, VRPosePtr pose);
+
+  // VRClientDispatcher implementation
   void OnDeviceChanged(VRDisplayPtr device) override;
 
  private:
@@ -51,6 +59,8 @@ class VRDeviceManager : public VRClientDispatcher {
   DEVICE_VR_EXPORT explicit VRDeviceManager(
       std::unique_ptr<VRDeviceProvider> provider);
 
+  VRDevice* GetDevice(unsigned int index);
+
   static void SetInstance(VRDeviceManager* service);
   static bool HasInstance();
 
@@ -73,6 +83,10 @@ class VRDeviceManager : public VRClientDispatcher {
   using ServiceList = std::vector<VRServiceImpl*>;
   ServiceList services_;
 
+  // Only one service and device is allowed to present at a time.
+  VRServiceImpl* presenting_service_;
+  VRDevice* presenting_device_;
+
   // For testing. If true will not delete self when consumer count reaches 0.
   bool keep_alive_;