Added polyfill to immersive-ar sample for consistency

Author: toji

proposals/immersive-ar-session.html

@@ -56,6 +56,14 @@
       import {SkyboxNode} from '../js/render/nodes/skybox.js';
       import {InlineViewerHelper} from '../js/util/inline-viewer-helper.js';
       import {Gltf2Node} from '../js/render/nodes/gltf2.js';
+      import {QueryArgs} from '../js/util/query-args.js';
+
+      // If requested, use the polyfill to provide support for mobile devices
+      // and devices which only support WebVR.
+      import WebXRPolyfill from '../js/third-party/webxr-polyfill/build/webxr-polyfill.module.js';
+      if (QueryArgs.getBool('usePolyfill', true)) {
+        let polyfill = new WebXRPolyfill();
+      }
 
       // XR globals.
       let xrButton = null;