Bug 1248761 - Destroy VRHMDInfo objects on the compositor thread r=daoshengmu

- Fixes a crash caused when shutting down the Oculus SDK objects
  in a different thread than the one that was used to
  initialize them.

MozReview-Commit-ID: DbB2pXVoqxL

--HG--
extra : rebase_source : 1447c4d9fd7f29bc14b8f91c6f5cd10dc1ec1f8b

gfx/vr/VRManager.cpp

@@ -85,6 +85,7 @@ VRManager::~VRManager()
 void
 VRManager::Destroy()
 {
+  mVRDevices.Clear();
   for (uint32_t i = 0; i < mManagers.Length(); ++i) {
     mManagers[i]->Destroy();
   }