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/202911Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>