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/+/1603474
Reviewed-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}
1 file changed