Add a function to check for the Ozone back end for use in skipping tests.

BUG=angleproject:1423

Change-Id: Ida3313e8cce5179422bdca85ba68f9765dfac840
Reviewed-on: https://chromium-review.googlesource.com/469068Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>