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/427307Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>