OutputHLSL was become a large, unweildy file. Some were also useful
to other classes, even on the GL back-end, but were inacessible.

Refactoring patch only.

BUG=angle:466

Change-Id: Id216147122ca105c6ccdf0ba0c5f6c5038726965
Reviewed-on: https://chromium-review.googlesource.com/203459Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>