Refactor validation of glGetSynciv. BUG=angleproject:1985 Change-Id: Idf1057e6d8d8c850643e3b2a22be5a881c8a2301 Reviewed-on: https://chromium-review.googlesource.com/474550 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>