Gather vertexStaticUse, fragmentStaticUse, and computeStaticUse
into a new mixin class StaticallyUsed, from which various
program resources can inherit to track their shader references.

BUG=angleproject:1920
Change-Id: Ibb3c0fe035bb8789aad65dbdfefaf7cb17b64c4e
Reviewed-on: https://chromium-review.googlesource.com/748317
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>