Bug 1343666 - Prevent crash on shutdown due to assertion in VRManagerChild::Get() r=daoshengmu

- Updated VREventObserver::DisconnectFromOwner() to call VRManagerChild::IsCreated()
  before calling the accessor VRManageRchild::Get() containing the assertion.

MozReview-Commit-ID: Ezy08m6V4HV

--HG--
extra : rebase_source : c6631dafa496cb03333ed417c14f1ff1692c8519

dom/vr/VREventObserver.cpp

@@ -45,8 +45,8 @@ VREventObserver::DisconnectFromOwner()
   mWindow = nullptr;
 
   // Unregister from VRManagerChild
-  VRManagerChild* vmc = VRManagerChild::Get();
-  if (vmc) {
+  if (VRManagerChild::IsCreated()) {
+    VRManagerChild* vmc = VRManagerChild::Get();
     vmc->RemoveListener(this);
   }
 }