Retry flaky webgl conformance test for CrOS

conformance/textures/video/tex-2d-luminance-luminance-unsigned_byte.html
is flaky on chromeos-amd64-generic-rel.

Bug: 1145830
Change-Id: I93efd5497d75903efe0e19557c8981ef4f130121
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522606


Auto-Submit: kylechar <kylechar@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824934}