Add a std140 padding helper class. Using a helper class keeps our main parser stateless as we define std140 structs which need padding. The only functional change should be that we no longer use a global counter for struct padding hidden variables, but a local padding per-struct. BUG=angle:466 Change-Id: I8b92d65884b86571c8b2f052b0cba6150a4bbab0 Reviewed-on: https://chromium-review.googlesource.com/202911 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>