Use async VR presentation code for XRSession

components/script/dom/xr.rs

@@ -95,9 +95,7 @@ impl XRMethods for XR {
         }
 
         let session = XRSession::new(&self.global(), &displays[0]);
-        // XXXManishearth we should actually xr_present() here instead of
-        // in XRSession::new, and resolve a promise based on it
-        promise.resolve_native(&session);
+        session.xr_present(promise.clone());
         promise
     }
 }