Also passes WEBGL_compressed_texture_s3tc_srgb conformance on WebGL 1/2 on DX11

BUG=angleproject:1553
BUG=chromium:630498

Change-Id: If1d17b54b1e8b998410079fd217626410015d7f1
Reviewed-on: https://chromium-review.googlesource.com/422585
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>