Fix incorrect NPOT availability check. Add tests for enabling NPOT extensions. BUG=angleproject:1678 Change-Id: Ibcbfc1192bceb634deb2904dbb9644902471e3fd Reviewed-on: https://chromium-review.googlesource.com/425713 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>