Ensure sample pages track/update when devices are added/removed

Currently, the test pages only check once (on page load) if the specific
session type is supported. If a device is disconnected while a user is
on the page, the "Enter VR" button does not update with that change.
This change enables support within the button to listen to device change
events and update it's state based on whether the specified session
types are supported, and changes all of the pages to consume this new
option.

Given the nature of xr-barebones, it implements this directly as well
rather than relying on the button.

Bug: 873409
Change-Id: I4be8a50f88f796b2e4d7d9f0834d4d77cda7d586
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554643
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648354}
23 files changed