Fix shaders with invariant keyword. We would trigger assertion failures in Debug mode, and fail to parse and translate correctly in Release. BUG=angle:711 Change-Id: Ibb7f33b288376617598578f48c7bbdbdec044279 Reviewed-on: https://chromium-review.googlesource.com/210822 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org>