The shader translator is intended to prune unreferenced variables in
the future. To maintain test coverage when this is done, most tests
that used to have unreferenced GLSL locals and globals are edited to
use built-ins or reference the necessary variables instead.

BUG=angleproject:2166
TEST=angle_unittests, angle_end2end_tests

Change-Id: I3964e7a80d52fc04a95f57e73da6095e433095e8
Reviewed-on: https://chromium-review.googlesource.com/768740Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>