Vulkan: use "undefined" for layerProvokingVertex Use "undefined" for layerProvokingVertex because different platforms has different behavior on this. APP would have to set gl_Layer identically. Bug: angleproject:7185 Change-Id: Ia5c31f617c2441c1a6ac8d682c9ce98f464045e0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3592274 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>