AR sample: fix for webxr test pages when served from a GCP bucket

When sample is served from a bucket, the URL of the xrray polyfill
is incorrect.

TBR=bajones@chromium.org

Change-Id: I923aad855e2c8cf0353773441f4d20da4076dcab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1603474Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Auto-Submit: Piotr Bialecki <bialpio@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658314}

third_party/webxr_test_pages/webxr-samples/js/cottontail/src/scenes/scene.js

@@ -24,7 +24,7 @@ import {StatsViewer} from '../nodes/stats-viewer.js';
 import {Node} from '../core/node.js';
 import {vec3, quat} from '../math/gl-matrix.js';
 
-import '/js/xrray-polyfill.js';
+import '../../../../js/xrray-polyfill.js'
 
 export class WebXRView extends RenderView {
   constructor(view, pose, layer) {