Vulkan: Update build files after roll. They have consolidated to one helper script generator. Simplifies the GN build. This also re-enables the Vulkan back-end after the roll. BUG=angleproject:1319 Change-Id: Ie98568ae5d1dc46932ea8d698809561da7591836 Reviewed-on: https://chromium-review.googlesource.com/427307 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>