Add WebXR samples fork to third_party to keep in sync with Chrome

This change introduces a snapshot of the Immersive Web's samples
that we will modify to create test pages for Chrome.

Bug: 923149
Change-Id: Ibae3e183729c90193b989e9b36d9bca037e67685
Reviewed-on: https://chromium-review.googlesource.com/c/1413616
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625022}
80 files changed