Refactor StaticallyUsed 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>