Prevent new events from being sent to WebVR pages that are being closed.

device/vr/vr_device.cc

Index: device/vr/vr_device.cc
diff --git a/device/vr/vr_device.cc b/device/vr/vr_device.cc
index 78091044558e47db9ecb90ea30803c48fcf599ba..31e2a9f224ba78bdbca239298ea2a65f5ab1dbf4 100644
--- a/device/vr/vr_device.cc
+++ b/device/vr/vr_device.cc
@@ -31,9 +31,9 @@ void VRDevice::AddService(VRServiceImpl* service) {
 }
 
 void VRDevice::RemoveService(VRServiceImpl* service) {
+  displays_.erase(service);
   if (IsPresentingService(service))
     ExitPresent();
-  displays_.erase(service);
 }
 
 bool VRDevice::IsAccessAllowed(VRServiceImpl* service) {