Address review comments

src/core/scene/a-scene.js

@@ -56,6 +56,7 @@ class AScene extends AEntity {
     self.systems = {};
     self.systemNames = [];
     self.time = self.delta = 0;
+    self.usedOfferSession = false;
 
     self.behaviors = {tick: [], tock: []};
     self.hasLoaded = false;
@@ -270,14 +271,15 @@ class AScene extends AEntity {
    * @param {bool?} useAR - if true, try immersive-ar mode
    * @returns {Promise}
    */
-  enterVR (useAR, useOfferSession = false) {
+  enterVR (useAR, useOfferSession) {
     var self = this;
     var vrDisplay;
     var vrManager = self.renderer.xr;
     var xrInit;
 
     // Don't enter VR if already in VR.
-    if (navigator.xr.offerSession === undefined) { return Promise.resolve('OfferSession is not supported.'); }
+    if (useOfferSession && navigator.xr.offerSession === undefined) { return Promise.resolve('OfferSession is not supported.'); }
+    if (self.usedOfferSession) { return Promise.resolve('OfferSession was already called.'); }
     if (this.is('vr-mode')) { return Promise.resolve('Already in VR.'); }
 
     // Has VR.
@@ -296,9 +298,11 @@ class AScene extends AEntity {
         xrInit = this.sceneEl.systems.webxr.sessionConfiguration;
         return new Promise(function (resolve, reject) {
           var requestFunction = useOfferSession ? navigator.xr.offerSession.bind(navigator.xr) : navigator.xr.requestSession.bind(navigator.xr);
+          self.usedOfferSession |= useOfferSession;
           requestFunction(xrMode, xrInit).then(
             function requestSuccess (xrSession) {
               self.xrSession = xrSession;
+              self.usedOfferSession = false;
 
               vrManager.layersEnabled = xrInit.requiredFeatures.indexOf('layers') !== -1;
               vrManager.setSession(xrSession).then(function () {