Because our output signature is only dependent on the arrangment
of the attachments, not the attachment type, use the output
layout key for now. If we need to, we could store both, in the
future.

BUG=angle:705

Change-Id: I3b99954d30b91a4741fdd6f48f8ffcf88c0bea7a
Reviewed-on: https://chromium-review.googlesource.com/215846Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>