GitLab

Since the other similar condition to this in LayerTreeHost is a DCHECK,
and since this one only seems to be hit on shutdown, let's make this a
DCHECK.

TBR=bokan@chromium.org

Bug: 845097, 845097
Change-Id: If4fd880076f9ddd0c16a8bf5e63a76bd2b2a69fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598424Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657300}