Bug 1586110 - VRManager should only be called in parent process or GPU process. r=kip

Differential Revision: https://phabricator.services.mozilla.com/D48127

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

gfx/vr/VRManager.cpp

@@ -78,6 +78,7 @@ static StaticRefPtr<VRManager> sVRManagerSingleton;
 /* static */
 VRManager* VRManager::Get() {
   MOZ_ASSERT(sVRManagerSingleton != nullptr);
+  MOZ_ASSERT(XRE_IsParentProcess() || XRE_IsGPUProcess());
 
   return sVRManagerSingleton;
 }