Invariant declarations didn't affect static use before, but now they
are also skipped in CollectVariables so an invariant declaration is
not enough in itself to mark a variable as active. This fixes an
assert in CollectVariables.

BUG=chromium:829553
TEST=angle_unittests

Change-Id: I3e51d2916f091bcc283af136a4abc846ff71447d
Reviewed-on: https://chromium-review.googlesource.com/999532Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>