This will make it easier to insert internal uniform layouts in Vulkan.

Bug: angleproject:2717
Change-Id: Ic1a76848337cc67a57698913c584d8596bf4f27e
Reviewed-on: https://chromium-review.googlesource.com/1127300Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>