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-by: Nico Weber <>
Reviewed-by: Klaus Weidner <>
Commit-Queue: Bill Orr <>
Cr-Commit-Position: refs/heads/master@{#625022}
80 files changed