Clarify the code so that it will be easier to add marking statically
written variables here.

BUG=angleproject:2262
TEST=angle_unittests

Change-Id: I821bde29beb89e0f3b0f99dc187d2840f8cd0f9b
Reviewed-on: https://chromium-review.googlesource.com/977963
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>