Bug 1540590 - Moving VRManager::Shutdown() task to Compositor thread to avoid race conditions. r=kip

Differential Revision: https://phabricator.services.mozilla.com/D25829

--HG--
extra : moz-landing-system : lando

gfx/vr/ipc/VRGPUChild.cpp

@@ -53,7 +53,10 @@ void VRGPUChild::Shutdown() {
 
 void VRGPUChild::ActorDestroy(ActorDestroyReason aWhy) {
   VRManager* vm = VRManager::Get();
-  vm->Shutdown();
+  CompositorThreadHolder::Loop()->PostTask(NewRunnableMethod(
+    "VRGPUChild::ActorDestroy",
+    vm, &VRManager::Shutdown));
+
   mClosed = true;
 }