DOM Overlay available for experiments

A quick status update on the DOM Overlay proposal's status.

In case you haven't seen it yet, please see the draft spec at
https://github.com/immersive-web/dom-overlays . There aren't any
outstanding pull requests at this time, and the open issues are largely
about clarifications and extensions for headsets, so please take a look and
let us know what you think about the current state.

Chrome currently supports this as an experimental API, and as of
yesterday's Canary 81.0.4029.2 the implementation should match the draft
spec. It requires an ARCore-compatible Android phone
<https://developers.google.com/ar/discover/supported-devices> and enabling
"WebXR AR Module" and "WebXR Incubations" in chrome://flags.

There's a test page at
https://storage.googleapis.com/chromium-webxr-test/latest.html?target=proposals/phone-ar-hit-test.html?debugSources=true%26iframe=true
which includes a visualization of XR events and input source status, best
viewed in landscape mode. The reddish square at the bottom right is a
cross-origin iframe to demonstrate how that affects input events. (The
draft spec has a section discussing that.)

A slightly more fun example is this remote control car
<https://klausw.github.io/a-frame-car-sample/index.html>, that uses DOM
Overlay for an onscreen joystick and exit button.