Add polyfill for XRRay type

Most of the code for the polyfill was copy-pasted from native xr_ray.cc
& ported to JavaScript.

Bug: 960159
Change-Id: I7c8bb66bf1a635566dd82f423587c4b146efd8d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600407
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Auto-Submit: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657997}