This will make it easier to add tracking of static reads.

BUG=angleproject:2262
TEST=angle_unittests

Change-Id: I23e0587c022c45ac62434a2a0d8170e8978d1c24
Reviewed-on: https://chromium-review.googlesource.com/983916Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>