We were using the same list in the shader varying sorting
as well as in the varying packing validation in the
translator.

BUG=angle:466

Change-Id: Ic11758288e7a522d7c18a293de9e137e487e3978
Reviewed-on: https://chromium-review.googlesource.com/205595Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>