Automate WebXR head pose test

Automates the manual test for ensuring that head poses provided by
OpenVR are properly piped through Chrome and surfaced in WebXR.

Bug: 914895
Change-Id: I346fa8d40a4a0c9f265ac31d2ee69947ed1b5f13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589291
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655114}