This will make staging texture tracking easier.

BUG=angleproject:2034

Change-Id: I2f0418cc5784100bf6fba59b4bda1b2dd9dc18d1
Reviewed-on: https://chromium-review.googlesource.com/503251Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>