Validate gl_FragData and gl_FragColor access after parsing After this simply declaring both variables invariant is not treated as static use. This simplifies ParseContext a bit, but the effect on compiler performance tests seems marginal. BUG=angleproject:2450 TEST=angle_unittests Change-Id: Ib90cb1d2bd1331542d1cd37732f24efb7833036a Reviewed-on: https://chromium-review.googlesource.com/992112 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>