Vulkan: support depth offset. Update pipeline description with depth offset state. Enable test. BUG=angleproject:2353 Change-Id: Ib087bbff8e145b0e6e862eb6616a4943a989a7b2 Reviewed-on: https://chromium-review.googlesource.com/1173447 Commit-Queue: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>