Simplifies a lot of logic when we don't have to maintain mappings between
the driver and gl-layer locations.

BUG=angleproject:882
BUG=angleproject:1123

Change-Id: Ia94257a322f768fdfa3167000a46a0715820ef4d
Reviewed-on: https://chromium-review.googlesource.com/295231Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>