Add barebones AR w/ optional DOM overlay, fix barebones XR

The barebones XR sample was missing a negation, keeping the
start button disabled unexpectedly.

Copy the (fixed) barebones XR sample to create a barebones
AR sample with an optional DOM Overlay.

Also add a bit of CSS to make the barebones "enter XR" button
a bit easier to hit (and less ugly).

Change-Id: Ia2750b4c83d75d3f1faf3628a8593c9c78960442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016357
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Auto-Submit: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734995}
4 files changed