This also simplifies a bunch of code in backends that was used to handle
getting the indexFormat from the pipeline "late".
Bug: dawn:502
Change-Id: Ibae50c8df21323fd391515f6036552e9fb868d93
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/32023
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>