[ar] Close deferred sessions request when destroying AR device

Otherwise, we hit a DCHECK when navigating away from the AR page while
the AR session is being set up, e.g. while the AR DFM is installing.

Change-Id: Icb1c85410497a804f54346727b1caeefb7ab0275
Reviewed-on: https://chromium-review.googlesource.com/c/1395180
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620764}
1 file changed