Vulkan: Fix missing index buffer usage. The newer validation layers picked up this missing bit. BUG=angleproject:2237 Change-Id: I5bb2f1db3a4e284c003de0f5b23f7f613056af3a Reviewed-on: https://chromium-review.googlesource.com/760637 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>