This encapsulates expanding struct variables inside the VariablePacker
class that packs variables according to the GLSL ES spec.

The variable expansion step is no longer run twice when checking
uniforms against the max uniforms limit.

BUG=angleproject:2068
TEST=angle_unittests

Change-Id: I012ddaa249f71c0a78d937c98007c61352e64888
Reviewed-on: https://chromium-review.googlesource.com/608367
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>