Chrome Version: 69.0.3461.0
OS: Android 8.1

## What steps will reproduce the problem?

(1) Ensure WebXR+AR is enabled (latest canary, #webxr/#webxr-hit-test flags enabled, ARCore installed
(2) Go to https://ar-request-session-bug.glitch.me/
(3) Click "Enter AR"
(4) Around ~50% of the time, the call to `requestSession` fails throwing `DOMException: Request for exclusive XRSession was denied.` It's a magic window session (only `outputContext`), not an exclusive session, so this is an especially strange error to get. This was working in 69.0.3457.0.

## What is the expected result?

To not throw the error consistently.

## What happens instead?

The error is (sometimes) thrown. There seems to be no obvious reason why, I've seen it both fail and succeed on the same page depending on reloading, on both local and remote sites.

The same content was working consistently this morning with version 69.0.3457.0.